Harden MCP suggestion sampling

[Widthdom]

Summary

• Bound suggest_improvement sampling responses before parsing and parse sampled metadata JSON with an explicit depth limit.
• Bound outbound sampling prompts, clamp long prompt fields, and summarize toolInvocationContext without sending its raw content to the sampling client.
• Added focused MCP regression coverage for oversized responses, deep JSON responses, and prompt bounding/redaction.

Documentation and changelog

• Updated USER_GUIDE.md for bounded MCP sampling prompts.
• Added changelog fragments:
  • changelog.d/unreleased/2897.security.md
  • changelog.d/unreleased/2898.security.md

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~SuggestImprovement_WhenSamplingResponse" -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~SuggestImprovement_WhenSampling" -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet build CodeIndex.sln -p:UseSharedCompilation=false
• dotnet test CodeIndex.sln -p:UseSharedCompilation=false
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json

Review

Adversarial review (origin/main..HEAD): No blocking/actionable issues found.

Follow-up candidates

None.

Fixes #2897
Fixes #2898