Limit MCP response serialization

[Widthdom]

Summary

• Replace MCP response-size checks that called ToJsonString with a bounded UTF-8 JSON writer that stops when the configured byte cap is crossed.
• Apply the same bounded measurement to batch_query response estimates and mark whether response_too_large.error.data.actual_bytes is exact.
• Add MCP tests for early bounded serialization and the additive exactness flag.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter FullyQualifiedName~McpServerTests -p:UseSharedCompilation=false -m:1 -nr:false
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet format CodeIndex.sln --verify-no-changes --no-restore
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json

Attempted dotnet test CodeIndex.sln -p:UseSharedCompilation=false; sandboxed MSBuild first failed with named-pipe bind permission errors, then the escalated full run surfaced an unrelated transient SQL extractor test failure that passed when rerun directly and was stopped after becoming long-running. Targeted MCP coverage passed on net8.0 and net9.0 after the final rebase-equivalent cleanup.

Documentation and Changelog

• Added changelog.d/unreleased/2860.security.md.
• No docs update was needed because the configured MCP response limit remains the same; this changes enforcement timing and adds response error metadata.

Review

• Codex adversarial review via .codex/workflows/adversarial-review.md: no blocking/actionable issues found.

Follow-up Candidates

• None.

Fixes #2860