Cap MCP audit and client response payloads

[Widthdom]

Summary

• Cap MCP audit argument key metadata with explicit omitted/truncated counters.
• Reject oversized MCP out-of-band client result/error payloads before cloning or retaining them.
• Serialize MCP audit log records through a per-record byte cap and mark reduced records with event_truncated.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~Issue3105" -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~Issue3098" -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~Issue3107" -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~McpServerTests|FullyQualifiedName~McpAuditLogTests|FullyQualifiedName~AuditLogSinkTests" -p:UseSharedCompilation=false
• dotnet build CodeIndex.sln -p:UseSharedCompilation=false
• dotnet format CodeIndex.sln --verify-no-changes
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• adversarial review of origin/main..HEAD: No blocking/actionable issues found.

Documentation and Changelog

• Added changelog.d/unreleased/3098.security.md
• Added changelog.d/unreleased/3105.fixed.md
• Added changelog.d/unreleased/3107.fixed.md
• No AGENTS.md, CLAUDE.md, or AGENT_GUIDE.md changes.

Follow-up Candidates

• None.

Fixes #3107
Fixes #3105
Fixes #3098