Bound and sanitize TestTelemetry TRX processing

[Widthdom]

Summary

• Bound TestTelemetry TRX traversal, file discovery, file size, XML parser work, and retained result aggregation.
• Sanitize TRX diagnostics to use relative paths and stable reason codes instead of raw XML/parser messages.
• Skip non-regular .trx filesystem entries and discard malformed TRX files atomically.

Validation

• dotnet build
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~TestTelemetryTests" --no-restore -p:UseSharedCompilation=false
• dotnet test CodeIndex.sln --no-restore -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet format CodeIndex.sln --verify-no-changes
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json

Documentation and changelog

• Added changelog.d/unreleased/3442.security.md.
• Added changelog.d/unreleased/3443.security.md.
• No README/developer guide update was needed because the changed behavior is limited to the internal test telemetry helper.

Review

• Codex adversarial review found incomplete traversal bounds, partial malformed-XML aggregation, and non-regular .trx entry handling risks.
• The findings were addressed with additional TestTelemetry should bound TRX discovery, XML parsing, and in-memory result collection #3442 commits and focused tests.

Follow-up candidates

• None.

Fixes #3442
Fixes #3443