Fix MCP JSON-RPC input handling

[Widthdom]

Summary

• Return spec-compliant MCP parse errors with id: null and bounded JSON depth.
• Enforce MCP frame character/UTF-8 byte limits and surface them in MCP status.
• Add JSON-RPC batch array support with notification-only, empty, nested, scalar-item, and invalid-item behavior covered.

Documentation / Changelog

• Updated DEVELOPER_GUIDE.md MCP transport notes.
• Added changelog.d/unreleased/1816.fixed.md.
• Added changelog.d/unreleased/1799-1996.fixed.md.
• Added changelog.d/unreleased/1892.added.md.

Fixes #1816
Fixes #1799
Fixes #1996
Fixes #1892

Validation

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

Review

• Codex adversarial review found a batch invalid-item response gap; fixed in commit 166cbb5f6.
• Later review attempts were affected by origin/main advancing during the run and reported unrelated reverse diffs; branch was merged with latest origin/main and revalidated.

Follow-up candidates

• None.