Add query observability instrumentation

[Widthdom]

Summary

• Add opt-in ActivitySource tracing for MCP requests and tracked SQLite queries.
• Accept MCP params._meta.traceparent as parent W3C trace context.
• Add CDIDX_SLOW_QUERY_MS stderr slow-query diagnostics and document query profiling controls.

Documentation and changelog

• Updated DEVELOPER_GUIDE.md with the CodeIndex ActivitySource name and profiling controls.
• Added changelog.d/unreleased/1679.added.md.
• Added changelog.d/unreleased/1786.fixed.md.
• Added changelog.d/unreleased/1789.fixed.md.

Fixes #1679
Fixes #1786
Fixes #1789

Validation

• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -p:UseSharedCompilation=false --filter "FullyQualifiedNameDbDebugTests.ExecuteTrackedReader_EmitsActivityAndSlowQueryLog|FullyQualifiedNameMcpServerTests.ProcessFrame_UsesTraceParentFromMetaAsActivityParent|FullyQualifiedName~McpServerTests.ProcessFrame_IgnoresNonStringTraceParent"
• dotnet run --project tools/CodeIndex.Changelog -- check

Review

• Adversarial review found traceparent type handling and duplicate slow-query logging issues; both were fixed.

Follow-up candidates

• None.