Migrate testing to native Microsoft.Testing.Platform and update test packages

[ptr727]

Unblocks the Dependabot nuget PRs (#545, #546) and every later nuget bump.

Stacked on #547, so this PR is based on resync/eol-lf and will retarget to
develop automatically once #547 merges. Its own diff is the five files below.

Reworked after ptr727/ProjectTemplate#1111 merged. The first revision used
coverlet.MTP, which was the open recommendation at the time. The hub has since
settled WORKFLOW.md D1.6 on Microsoft.Testing.Extensions.CodeCoverage, and this
PR now follows that instead.

The problem

xunit.v3 4.0.0 removed the VSTest bridge, so the CI unit-test step's
dotnet test --collect:"XPlat Code Coverage" now fails outright on the .NET 10
SDK:

error : Testing with VSTest target is no longer supported by Microsoft.Testing.Platform
on .NET 10 SDK and later. If you use dotnet test, you should opt-in to the new
dotnet test experience.

That is what #545 and #546 have been red on, and it blocks every later bump until
the runner moves.

The fix, per WORKFLOW.md D1.6

Change	Why
global.json with {"test":{"runner":"Microsoft.Testing.Platform"}}	opts into the native MTP runner. No sdk section, so SDK resolution and roll-forward are untouched.
drop xunit.runner.visualstudio	the VSTest adapter MTP replaces
coverlet.collector -> Microsoft.Testing.Extensions.CodeCoverage 18.9.0	coverlet's VSTest data collector is ignored under MTP without failing
CI step -> dotnet test --coverage --coverage-output-format cobertura --results-directory ./coverage, then prefix each report to coverage-<guid>.cobertura.xml	matches the hub's validate-task.yml byte for byte

Three details are load-bearing, and none of them reds the job on its own:

• The 18.9.0 floor. Below 18.1.0 the extension is built against
  Microsoft.Testing.Platform 1.x and throws TypeLoadException against the 2.x
  platform xunit.v3 4.0.0 carries. It then runs zero tests and still writes a
  well-formed Cobertura file reporting full coverage.
• --coverage-output stays unset. Pinning one filename would give every test
  project in a solution the same path, and the last to finish would overwrite the
  rest.
• The prefix rename. The default <guid>.cobertura.xml that the unset flag
  produces is a name codecov-cli's finder does not match (its patterns are
  *coverage*.* and an exact cobertura.xml), so an unprefixed report uploads
  nothing while the step still exits green.

Package bumps the runner change unblocks: AwesomeAssertions 9.5.0 -> 9.6.0,
xunit.analyzers 1.27.0 -> 2.0.0, xunit.v3 3.2.2 -> 4.0.0.
Microsoft.NET.Test.Sdk stays at 18.9.0 (already current).

.gitignore gains the hub's dotnet coverage block verbatim. The output was
untracked and unignored, so a blanket git add -A after a local coverage run
would have staged it.

Verification

Against the real invocation, not the documented one:

• 21 tests ran and passed, which is the check that matters given the
  zero-test failure mode above.
• Resolved graph (from obj/project.assets.json, not the csproj text):
  Microsoft.Testing.Extensions.CodeCoverage/18.9.0,
  Microsoft.Testing.Platform/2.3.3, xunit.v3/4.0.0 with the mtp-v2 variants.
  No coverlet, no xunit.runner.visualstudio.
• The run wrote bbfde807-....cobertura.xml and the prefix step renamed it to
  coverage-bbfde807-....cobertura.xml, confirming the rename is genuinely
  needed rather than defensive.
• git check-ignore covers both filename shapes;
  git ls-files -z | xargs -0 git check-ignore -v is empty.
• validate-task.yml is the only dotnet test caller; publish-release.yml and
  test-pull-request.yml both reach it via uses:, so the publish gate and the
  PR gate move together. No --collect survives anywhere.
• Build, CSharpier, dotnet format style --verify-no-changes,
  editorconfig-checker, actionlint, markdownlint and cspell all clean.

One thing worth knowing: the reported coverage number moves, because the
engine does. Line rate goes from 0.59 under coverlet to 0.26 here, with
lines-valid 1350 -> 3143, since this engine instruments more of the graph. It
cannot gate anything: codecov.yml sets informational: true on both project
and patch, and the only ruleset-required check is
Check pull request workflow status job.

Notes

• Dependabot dual-targets develop and main, so Bump AwesomeAssertions and 3 others #546 (the main copy) stays
  blocked until this reaches main via the promotion PR. main's tree is not
  broken in the meantime, it just cannot take the bump.
• The fleet audit flags this repo's local validate-task.yml as hub-only.
  Now that Promote develop to main ProjectTemplate#1111 has landed MTP support in the hub's
  reusable workflow, that migration is unblocked, but it is a separate interface
  change covering five workflow files and belongs in its own PR rather than here.

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[qodo-code-review]

PR Summary by Qodo

Migrate tests and coverage collection to native MTP

🐞 Bug fix ✨ Enhancement ⚙️ Configuration changes 📝 Documentation 🕐 20-40 Minutes

AI Description

• Opt into native MTP to restore xUnit v4 testing on .NET 10.
• Replace VSTest coverage collection with coverlet.MTP Cobertura output for Codecov.
• Upgrade test dependencies and ignore generated coverage artifacts.

Diagram

graph TD
  G["global.json"] --> D["dotnet test"] --> X["xUnit v4"] --> C["coverlet.MTP"] --> O["Cobertura output"] --> U["Codecov upload"]
  W["Validation workflow"] --> D

Loading

High-Level Assessment

The native MTP migration is the appropriate approach because xUnit v4 no longer supports the VSTest bridge on .NET 10. Pinning older xUnit packages or retaining the collector would only defer dependency upgrades or silently lose coverage; the selected coverlet.MTP integration preserves the existing Codecov workflow.

Files changed (5) +20 / -10

Bug fix (1) +4 / -2

validate-task.ymlRun CI tests and coverage through native MTP +4/-2

Run CI tests and coverage through native MTP

• Replaces the VSTest '--collect' invocation with coverlet.MTP arguments that emit Cobertura reports into the existing Codecov directory. Comments document why the native runner requires the new command.

.github/workflows/validate-task.yml

Documentation (1) +4 / -0

AGENTS.mdDocument the native MTP testing workflow +4/-0

Document the native MTP testing workflow

• Explains the repository-wide MTP runner selection, local test command, CI coverage command, and incompatibility of the former VSTest collector syntax.

AGENTS.md

Other (3) +12 / -8

.gitignoreIgnore generated Cobertura coverage artifacts +3/-0

Ignore generated Cobertura coverage artifacts

• Ignores the coverage output directory and both standard and timestamped Cobertura XML filenames produced by local MTP coverage runs.

.gitignore

CreateMatrixTests.csprojUpgrade tests and replace VSTest coverage packages +4/-8

Upgrade tests and replace VSTest coverage packages

• Upgrades AwesomeAssertions, xUnit, and xUnit analyzers. Removes the Visual Studio runner and VSTest coverlet collector in favor of the self-registering coverlet.MTP package.

CreateMatrixTests/CreateMatrixTests.csproj

global.jsonSelect Microsoft.Testing.Platform for dotnet test +5/-0

Select Microsoft.Testing.Platform for dotnet test

• Adds repository-wide test configuration that opts into the native Microsoft.Testing.Platform runner without constraining SDK selection or roll-forward behavior.

global.json

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.01%. Comparing base (2efaeb2) to head (dc48c3b).

Additional details and impacted files

@@                Coverage Diff                @@
##           resync/eol-lf     #549      +/-   ##
=================================================
- Coverage          57.28%   57.01%   -0.27%     
=================================================
  Files                 15       15              
  Lines               1386     1375      -11     
  Branches              89      108      +19     
=================================================
- Hits                 794      784      -10     
  Misses               573      573              
+ Partials              19       18       -1     

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[Copilot]

🟢 Approval recommended

The changes are small, consistent across docs/tests/CI, and fully remove the incompatible VSTest-based coverage invocation needed for .NET 10 + xunit.v3 4.0.0.

Pull request overview

This PR migrates the repository’s unit test execution from the legacy VSTest-based flow to the native Microsoft.Testing.Platform runner (required for .NET 10 + xunit.v3 4.0.0), unblocking current and future Dependabot NuGet upgrades and restoring CI coverage generation.

Changes:

• Opt into the Microsoft.Testing.Platform runner via global.json.
• Update the test project’s package set for xUnit v3 4.0.0 compatibility (drop VSTest adapter/collector; use coverlet.MTP).
• Update CI to run dotnet test with Coverlet MTP arguments and ignore generated coverage artifacts in git.

File summaries

File	Description
global.json	Opts dotnet test into the native Microsoft.Testing.Platform runner.
CreateMatrixTests/CreateMatrixTests.csproj	Updates test dependencies for xUnit v3 4.0.0 and switches coverage to coverlet.MTP.
AGENTS.md	Documents the repo’s testing/coverage invocation under the native MTP runner.
.gitignore	Prevents accidental staging of generated coverage output files.
.github/workflows/validate-task.yml	Replaces --collect (VSTest) with MTP-compatible Coverlet flags and preserves Codecov ingestion path.

Review details

• Files reviewed: 4/5 changed files
• Comments generated: 0
• Review effort level: Lite

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[Copilot]

🟢 Approval recommended

The runner/package/workflow changes are internally consistent (docs, CI invocation, and ignores match) and no remaining --collect/coverlet adapter usage is present in the repo.

Review details

• Files reviewed: 4/5 changed files
• Comments generated: 0 new
• Review effort level: Lite

[ptr727]

Superseded by #553.

GitHub closed this automatically when its base branch resync/eol-lf was deleted as part of merging #547. That was my sequencing error: with a stacked set, the children have to be retargeted before the parent is merged with --delete-branch, and I did it the other way round. Reopening is not possible once the base branch is gone.

#553 is the same branch and the same content, targeting develop directly. The review history here still stands: this PR reached CLEAN with zero findings across two rounds, and nothing about the change has altered since.