Honor Git helper cancellation tokens

[Widthdom]

Summary

• Pass caller cancellation tokens through Git-backed freshness, status, repository metadata, and commit-resolution paths.
• Preserve cancellation propagation through best-effort helpers and status database handling.
• Add regression coverage and a bilingual changelog fragment for Require explicit cancellation tokens on Git helper execution paths #3723.
• Stabilize the MCP timeout diagnostics regression test after status diagnostics started observing request cancellation.

Validation

• dotnet build CodeIndex.sln --no-restore
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --framework net8.0 --filter "FullyQualifiedName~McpServerTests.ProcessFrameAsync_RequestTimeout_ReturnsStructuredTimeoutError"
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~RequestTimeout_ReturnsStructuredTimeoutError"
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --no-build
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet format CodeIndex.sln --verify-no-changes --no-restore
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll index . --commits HEAD --json
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• Codex adversarial review: No blocking/actionable issues found.

Fixes #3723