Stabilize C# regex allocation regression probes under full-suite load

[Widthdom]

Summary

The net9.0 Release full suite can fail SymbolExtractorCSharpRegexProbeTests.Extract_CSharpPropertyStructuralGate_PreservesPropertyFormsAndReducesAllocations when the optimized and baseline allocation measurements are very close, while the same test passes immediately when rerun in isolation.

Evidence

Full-suite command:

dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -f net9.0 --no-build --no-restore -m:1 -nodeReuse:false

Observed failure:

Expected property structural gating to allocate less: optimized=5,805,248, baseline=5,798,592 bytes.

The full suite completed with 11,644 passed, 431 skipped, and this single failure. An immediate isolated rerun under the same Release/net9.0 configuration passed:

dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -f net9.0 --filter FullyQualifiedName~Extract_CSharpPropertyStructuralGate_PreservesPropertyFormsAndReducesAllocations --no-build --no-restore -m:1 -nodeReuse:false

Expected

The allocation regression probe should distinguish a material regression from measurement noise under full-suite load, while continuing to fail for a meaningful optimized-path allocation increase.

Scope

Stabilize the measurement or comparison contract and retain property-form correctness coverage. This was observed while validating unrelated Issue #5229 and is not part of that implementation.

[Widthdom]

Additional full-suite occurrence on the latest origin/main integration:

dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -f net8.0 --no-build --no-restore -m:1 -nodeReuse:false

SymbolExtractorCSharpRegexProbeTests.Extract_CSharpWrappedModifierLookup_ReducesAllocationsOnRepeatedWrappedConstructors failed with:

Expected wrapped lookup caching to allocate less: optimized=2,015,848, baseline=1,989,144 bytes.

The full suite completed with 12,151 passed, 9 skipped, and this single failure. The exact test then passed immediately in isolation under the same Release/net8.0 configuration:

dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -f net8.0 --filter FullyQualifiedName~Extract_CSharpWrappedModifierLookup_ReducesAllocationsOnRepeatedWrappedConstructors --no-build --no-restore -m:1 -nodeReuse:false

This is the same full-suite allocation-measurement instability as the property structural-gate occurrence in the issue body, so it should be covered by the same stabilization work.