[release/11.0.1xx] Forward MSBuild -mt switches to the build engine

[JanProvaznik]

Backport of #56120 to release/11.0.1xx.

Cherry-picked the squash commit 6d10cf8ab0e932ec2efa5c74eb759ec75d06ae9c. This branch contains one commit on top of the release branch, without unrelated main history.

Release-specific adaptations

• Apply MTP argument routing in MSBuildUtility, where that logic lives on the release branch, rather than adding main's extracted TestCommandOptions class. Preserve arguments after -- and add coverage for that boundary.
• Retain the release branch's watch logging options. The combined logging/-mt regression uses --verbose; the unrelated main-only --trace feature is not backported.

Validation

• CLI/MTP/run parser tests: 124 passed, one platform-specific skip.
• Watch forwarding and file-discovery tests: 50 passed.

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[Copilot]

🔵 Needs a closer look

It changes core CLI build/forwarding behavior (run/test/watch) and introduces a new MSBuild command-line assembly reference path, which warrants final human validation for servicing-branch safety.

Pull request overview

Backports the fix to ensure MSBuild build-engine -mt/-multithreaded switches are treated as MSBuild-only and forwarded to the underlying build invocation (instead of leaking to launched applications), including correct handling across -- boundaries and dotnet-watch file discovery.

Changes:

• Extend MSBuild-only token separation (previously logger-focused) to also recognize -mt/-multithreaded and route these to MSBuild for dotnet run and dotnet test (MTP).
• Update dotnet-watch argument handling to exclude MSBuild-only switches from positional file discovery while still forwarding them to the internal dotnet build invocation.
• Factor the MSBuild command-line assembly reference logic into MSBuildRuntimeReference.targets and import it where needed; add regression coverage for quoting/response-file/-- cases.

File summaries

File	Description
test/dotnet.Tests/LoggerUtilityTests.cs	Adds coverage for recognizing -mt forms as MSBuild-only and preserves -- semantics for MTP.
test/dotnet.Tests/CommandTests/Run/RunParserTests.cs	Validates dotnet run keeps -mt on the MSBuild side before -- and treats it as app args after --.
test/dotnet.Tests/CommandTests/Run/RunFileTests_BuildOptions.cs	Adds end-to-end coverage for response files, env vars, and node-count behavior when -mt is used.
test/dotnet-watch.Tests/CommandLine/ProgramTests.GetProjectOptions.cs	Ensures file-based entry point discovery isn’t broken when -mt precedes the file argument.
test/dotnet-watch.Tests/CommandLine/CommandLineOptionsTests.cs	Adds watch forwarding/file-discovery coverage for -mt valid/invalid and -- boundary cases.
src/Dotnet.Watch/dotnet-watch/Program.cs	Switches file discovery parsing to use the new filtered argument list.
src/Dotnet.Watch/dotnet-watch/CommandLine/CommandLineOptions.cs	Filters MSBuild-only tokens (binlog + -mt) out of file-discovery args and forwards -mt to build args.
src/Cli/MSBuildRuntimeReference.targets	New shared target/import providing MSBuild command-line assembly reference + package path validation.
src/Cli/Microsoft.DotNet.Cli.Utils/Microsoft.DotNet.Cli.Utils.csproj	Replaces inlined MSBuild runtime reference logic with shared import.
src/Cli/Microsoft.DotNet.Cli.Definitions/Utilities/MSBuildArgumentParser.cs	Adds centralized parsing for recognizing valid -mt/-multithreaded switch forms (incl. MSBuild-style quoting).
src/Cli/dotnet/LoggerUtility.cs	Generalizes logger-only splitting into “MSBuild-only” splitting and includes -mt recognition.
src/Cli/dotnet/dotnet.csproj	Imports shared MSBuild runtime reference targets so dotnet can use MSBuild command-line internals.
src/Cli/dotnet/Commands/Test/MTP/MSBuildUtility.cs	Routes MSBuild-only tokens (incl. -mt) to MSBuild while preserving args after -- for the test app.
src/Cli/dotnet/Commands/Run/VirtualProjectBuildingCommand.cs	Applies MSBuild -mt/node-count to in-process build parameters (managed-only).
src/Cli/dotnet/Commands/Run/RunCommand.cs	Ensures MSBuild-only tokens don’t leak into application args for dotnet run.
src/Cli/dotnet/Commands/Publish/PublishCommand.cs	Renames transformer arg to reflect generalized MSBuild-only splitting.
src/Cli/dotnet/Commands/Pack/PackCommand.cs	Renames transformer arg to reflect generalized MSBuild-only splitting.
src/Cli/dotnet/Commands/DotNetCommandFactory.cs	Uses generalized MSBuild-only splitting so -mt doesn’t interfere with file-based program routing.
src/Cli/AGENTS.md	Documents the shared MSBuild command-line assembly reference mechanism and constraints.

Review details

• Files reviewed: 19/19 changed files
• Comments generated: 0
• Review effort level: Lite

[JanProvaznik]

checked offline that rc2 is not locked down, so removing the auto-applied servicing label