Improve MCP request correlation and telemetry

[Widthdom]

Summary

• Propagate MCP JSON-RPC request IDs and server correlation IDs into stderr diagnostics and structured error data.
• Echo correlation metadata on successful MCP responses and batch_query slot results.
• Emit redacted mcp.tool.invocation telemetry lines for MCP tools/call requests.

Validation

• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet test CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release --filter "FullyQualifiedName~McpServerTests|FullyQualifiedName~McpAuditLogTests" -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• Adversarial review: No blocking/actionable issues found.

Documentation and changelog

• Updated DEVELOPER_GUIDE.md with MCP request correlation and telemetry contracts.
• Added changelog fragments:
  • changelog.d/unreleased/1814.fixed.md
  • changelog.d/unreleased/1897.fixed.md
  • changelog.d/unreleased/1898.added.md

Follow-up candidates

• None.

Fixes #1814
Fixes #1897
Fixes #1898