Fix git fixture signing and stderr tee failures

[Widthdom]

Summary

• Disable commit/tag signing in Git-backed test fixture repositories so global signing settings cannot break non-interactive fixture commits.
• Harden persistent stderr tee logging so disposed console/log writers do not cascade ObjectDisposedException into callers.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~IndexCommandRunnerTests|FullyQualifiedName~GitHelperTests|FullyQualifiedName~GitTestProjectHelperTests"
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~GlobalToolLogTests|FullyQualifiedName~HttpMcpTransportTests"
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet test CodeIndex.sln -p:UseSharedCompilation=false

Documentation and Changelog

• Updated TESTING_GUIDE.md Git test guidance.
• Added changelog.d/unreleased/2729.fixed.md.
• Added changelog.d/unreleased/2730.fixed.md.

Review

• Manual adversarial review of origin/main..HEAD: no blocking/actionable issues found.
• Attempted required codex exec adversarial review, but it was blocked by Codex usage limits in this environment.

Follow-up

• Discovered full cdidx index stall/degraded-index validation issue: Full cdidx index can stall near JavaScriptTypeScriptSupport symbol extraction #2740.

Fixes #2729
Fixes #2730