LocalJsonl source guard can cross parallel unreadable test markers

[Widthdom]

Summary

LocalJsonlJsonWriterOptionsTests.RelaxedEncoderAndHelperStayLimitedToLocalJsonlSinks can fail during the full parallel Release suite when its repository-wide source enumeration crosses an unreadable marker directory created by another test under a generated tests/CodeIndex.Tests/bin/Debug/... tree.

Observed behavior

Command:

dotnet test CodeIndex.sln -c Release -p:UseSharedCompilation=false --no-restore --logger "console;verbosity=minimal"

The test failed in EnumerateSourceFiles with:

System.UnauthorizedAccessException: Access to the path
tests/CodeIndex.Tests/bin/Debug/net8.0/mcp_index_unreadable_marker_.../secret is denied.

The generated unreadable marker belonged to another parallel test. A separate full Release suite completed successfully before this independent adversarial-review run, so the failure depends on scheduling and transient generated state rather than the LocalJsonl contract itself.

Expected behavior

The repository-wide guard should deterministically inspect the intended source set during the normal parallel suite without traversing generated bin / obj artifacts or failing on transient unreadable test fixtures.

Acceptance criteria

• The source enumeration excludes generated build/test trees or otherwise handles concurrent inaccessible generated paths without hiding failures in the intended source set.
• The test remains exhaustive for the repository files that define the LocalJsonl contract.
• The normal parallel Release suite no longer depends on the timing of unreadable-marker fixtures.

Scope note

Observed during the required adversarial review for #5113 and unrelated to that bounded-regex test stabilization.