ci: shard the full-suite coverage run across a matrix

[JSONbored]

Summary

• validate-code's "Test with coverage" step was ~9-10 of the ~11 minutes a typical backend PR spent in CI (confirmed via real timing pulled from ~15 recent CI runs): vitest schedules whole test files atomically to workers, so a single job with --maxWorkers=4 (a standard GitHub-hosted runner's real CPU budget) is capped at 4-way parallelism no matter how many test files exist.
• Adds a new validate-tests job, matrixed 4 ways over vitest's own --shard=<i>/<n> flag, giving N*4-way parallelism instead of 4-way.
• Each shard uploads its own lcov.info + junit report to Codecov; multiple uploads for the same commit/PR are additive there (standard pattern for matrix-split suites).
• Sets COVERAGE_NO_THRESHOLDS=true per shard, wiring up vitest.config.ts's existing (previously dormant/unused) support for it -- a shard only exercises part of the tree, so the global 90% backstop threshold would otherwise always false-fail per shard.
• validate-code keeps everything else (drift checks, typecheck, build engine, worker runtime tests, MCP/miner build+pack, REES, UI toolchain) as a single job, since those are fast (~1 min total) and don't benefit from a matrix.
• This is a public repo, so standard GitHub-hosted runners are free regardless of job count -- this costs nothing extra, only wall-clock time drops.
• Updates the two workflow-introspection tests (codecov-policy.test.ts, workflow-runner-labels.test.ts) that asserted the old single-job shape.

Why

Investigated at the user's request: CI validate was taking 10+ minutes per PR. Real timing data from recent runs showed ~85-95% of that was the single unsharded coverage step; everything else (path-filtered drift checks, typecheck, UI toolchain) was already fast thanks to the existing changes/paths-filter job.

Best paired with #4814 (splitting the two oversized test files) for balanced shards, but works standalone too.

Test plan

• npx tsc --noEmit clean
• npm run actionlint clean
• YAML parses (python3 -c "import yaml; yaml.safe_load(...)")
• Verified --shard=<i>/<n> + COVERAGE_NO_THRESHOLDS=true locally: threshold failure reproduces without the flag, disappears with it, and still produces a valid coverage report
• Updated codecov-policy.test.ts / workflow-runner-labels.test.ts assertions now pass against the new job shape

[superagent-security]

Superagent didn't find any vulnerabilities or security issues in this PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.23%. Comparing base (fe592b8) to head (0194dce).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4815   +/-   ##
=======================================
  Coverage   94.23%   94.23%           
=======================================
  Files         442      442           
  Lines       38845    38845           
  Branches    14150    14150           
=======================================
  Hits        36607    36607           
  Misses       1577     1577           
  Partials      661      661           

Flag	Coverage Δ
shard-1	51.48% <ø> (?)
shard-2	45.40% <ø> (?)
shard-3	41.95% <ø> (?)
shard-4	52.00% <ø> (?)

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Warning

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⏸️ Gittensory review result - manual review recommended

_{Review updated: 2026-07-11 00:38:03 UTC}

3 files · 1 AI reviewer · 2 blockers · readiness 93/100 · CI green · unstable

⏸️ Suggested Action - Manual Review

• No linked issue detected — If this PR is intended to solve an issue, link it explicitly in the PR body.
• Maintainer requires a linked issue — Link the relevant issue (for example Closes #123) before opening the PR.

Review summary
This PR splits the dominant ~9-10min unsharded coverage step out of validate-code into a new 4-way matrixed validate-tests job using vitest's native --shard flag, moving Codecov uploads to per-shard flags and updating the two workflow-introspection tests to match the new job shape. The mechanical restructuring (job split, needs/if wiring, test assertions) is internally consistent and well-traced against the visible diff. The one real concern is that COVERAGE_NO_THRESHOLDS=true is now set unconditionally on every shard invocation (push and PR alike), which per the PR's own description disables vitest.config.ts's global coverage threshold check entirely — no job in this workflow ever runs with that backstop enabled anymore.

Blockers

• ci.yml (validate-tests 'Test with coverage' step): COVERAGE_NO_THRESHOLDS is now hardcoded to "true" for every shard on every trigger (push included), so the global vitest coverage-threshold backstop that previously ran in the single unsharded job is now never enforced anywhere in CI — only Codecov's patch-coverage gate remains, which is a narrower check (changed-lines only, not whole-repo floor); confirm an equivalent whole-repo floor is enforced elsewhere (Codecov project-level target, a merge-then-check step) or explain why dropping it is acceptable.

Nits — 6 non-blocking

• Each of the 4 validate-tests shards independently rebuilds the engine package and reinstalls node_modules on a cache miss, so cache-miss PRs pay ~4x the install/build cost that a single job did before — acceptable on free runners but worth a comment if wall-clock during cache misses becomes noticeable.
• No concurrency/cancel-in-progress group is visible for the new validate-tests matrix in the diff — confirm superseded runs on force-pushes actually get cancelled rather than piling up 4 extra jobs per push.
• PR description frames this as CI-only investigation 'at the user's request' but doesn't link a tracked issue — worth confirming this counts as maintainer-authorized operator work under the issue-linking policy, since it's otherwise unsolicited infra churn.
• Consider gating COVERAGE_NO_THRESHOLDS so at least the `push` trigger (or a periodic scheduled run) still executes an unsharded, threshold-enforced full suite, preserving the whole-repo floor separately from per-shard patch coverage.
• codecov-policy.test.ts and workflow-runner-labels.test.ts updates look correctly re-pointed at validate-tests — no fabricated assertions spotted, but worth adding one test asserting COVERAGE_NO_THRESHOLDS is actually read somewhere (vitest.config.ts) to close the loop this PR claims to wire up.
• Diff looks like trivial or whitespace-only churn — Reduce whitespace-only or formatting-only churn and keep the diff focused on substantive changes.

Concerns raised — review before merging

• No linked issue detected — If this PR is intended to solve an issue, link it explicitly in the PR body.
• Maintainer requires a linked issue — Link the relevant issue (for example Closes #123) before opening the PR.

Signal	Result	Evidence
Code review	❌ 2 blockers	1 reviewer
Linked issue	⚠️ Missing	No linked issue or no-issue rationale found.
Related work	✅ No active overlap found	No same-issue or scoped active PR overlap found.
Change scope	✅ 20/20	Low review scope from cached public metadata (no linked issue context).
Validation posture	✅ 25/25	PR body includes validation/test evidence.
Contributor workload	✅ 10/10	Author activity: 48 registered-repo PR(s), 40 merged, 275 issue(s).
Contributor context	✅ Confirmed Gittensor contributor	JSONbored; Gittensor profile; 48 PR(s), 275 issue(s).
Gate result	❌ Blocking	Repo-configured hard blocker found.

Review context

• Author: JSONbored
• Role context: owner (maintainer lane)
• Public audience mode: oss maintainer
• Lane context: Repository is configured for direct PR review.
• Public profile languages: not available
• Official Gittensor activity: 48 PR(s), 275 issue(s).
• PR-specific overlap: none found.

Contributor next steps

• Treat this as maintainer-lane context rather than normal contributor-lane activity.
• Explain no-issue PR.
• Link the issue being solved, or explicitly explain why this is a no-issue PR.

Signal definitions

• Related work = same linked issue, overlapping active PRs, or title/path similarity.
• Change scope = cached public metadata such as size labels, draft state, and review-burden hints.
• Validation posture = whether the PR provides enough public validation/test evidence for maintainer review.
• Contributor workload = public contributor activity and cleanup pressure, not a repo-wide quality failure.
• Contributor context = public GitHub/Gittensor identity context; non-Gittensor status is not a blocker.

_{🟩 Safe / merged · 🟦 Advisory · 🟨 Held for review · 🟥 Blocked / closed}

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