Fix MCP batch query failure reporting

[Widthdom]

Summary

• Add top-level batch_query aggregate fields: total_count, success_count, failure_count, partial_failure, failure_scope, and cascade_started_at_index.
• Validate batched inner tool argument JSON types before dispatch, including limit, format, and other known argument shapes.
• Document the new batch_query response contract and add bilingual changelog fragments.

Validation

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

Documentation / Changelog

• Updated batch_query MCP tool description.
• Added changelog fragments:
  • changelog.d/unreleased/1472.fixed.md
  • changelog.d/unreleased/1615.fixed.md
  • changelog.d/unreleased/1616.fixed.md

Follow-up Candidates

• None.

Fixes #1472
Fixes #1615
Fixes #1616