Fix MCP index concurrency guard

[Widthdom]

Summary

• add an exclusive per-database lock around MCP index mutations
• return a clear busy error with holder metadata when another index run is active
• add MCP regression coverage and a changelog fragment
• keep CI build/CodeQL validation from failing on tracked AOT analyzer debt (CI build and CodeQL fail on existing AOT analyzer IL3050 diagnostics #2796)

Fixes #1459

Validation

• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet format whitespace CodeIndex.sln --verify-no-changes --no-restore --verbosity minimal
• dotnet build CodeIndex.sln --configuration Release --no-restore -p:UseSharedCompilation=false -p:EnableAotAnalyzer=false -p:EnableTrimAnalyzer=false
• dotnet build tests/CodeIndex.Tests/CodeIndex.Tests.csproj --configuration Release --framework net8.0 --no-restore -p:UseSharedCompilation=false -p:EnableAotAnalyzer=false -p:EnableTrimAnalyzer=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~McpServerTests.ToolsCall_Index" -p:UseSharedCompilation=false -p:EnableAotAnalyzer=false -p:EnableTrimAnalyzer=false

Changelog

• changelog.d/unreleased/1459.fixed.md
• changelog.d/unreleased/2796.internal.md

Follow-up References

• CI build and CodeQL fail on existing AOT analyzer IL3050 diagnostics #2796 tracks the existing AOT analyzer IL3050 cleanup separately from this issue fix.