test(forge): skip entries removed mid-walk in the owner sentinel scan (RIG-5408)

[rigel-mintaka]

Summary

Fixes RIG-5408. TestOwnerSentinelSingleChokepoint walks go/. Meanwhile guestd's TestExecResolvesRelativePATHFromWorkdir creates and deletes guest-relative-path-* inside the source tree. If the walk lists that dir and then opens it after deletion, it fails with ENOENT (seen on #2285, run 38407100289).

• The walk moves into sentinelFiles and skips fs.ErrNotExist, both from WalkDir and from the file read. The scan only checks non-test .go files, so a vanished entry never matters.
• New TestSentinelFilesSkipsEntriesRemovedMidWalk: deletes a listed directory and a listed file during the walk.

Test-only change.

Tests

• Red before the fix: sentinelFiles: open …/b: no such file or directory.
• Green after: both tests pass 20/20 (-count=20). The full go/internal/forge package passes.
• gofmt, go vet, golangci-lint (forge): clean.

[trunk-io]

😎 This pull request was merged.

[linear-code]

RIG-5408

[github-actions]

Compass engineering docs preview: https://compass-forge-rig-5408-senti.compass-eng-docs.pages.dev

Deployed from compass-forge/rig-5408-sentinel-walk-enoent at a11a855.

[rigel-mintaka]

Review result

• Reviewed SHA: a11a8558b7500a9ab6c34d926e25a293c906bbf0
• Base: main@origin b1e851565e7d
• Result: high 0 / medium 0 / low 0. All-clear.

Findings checked

• The regression test covers both ways an entry can vanish. While a.go is being read, the test removes b/ and c.go.
  • Removing b/ makes WalkDir's later ReadDir return fs.ErrNotExist.
  • Removing c.go makes the injected read return fs.ErrNotExist.
  • Dropping either skip makes the test fail. Without the fix it fails with open …/b: no such file or directory.
• A real chokepoint violation is still caught. Errors other than fs.ErrNotExist still fail the walk, and the scan still requires exactly owner.go. The only entries skipped are ones deleted during the run. Those are other tests' temp dirs, which contain no .go source.
• Verification:
  • Both tests pass 20/20 (-count=20), and the full go/internal/forge package passes.
  • gofmt, go vet and golangci-lint are clean.
  • CI is green on this head.