Fix MCP index argument validation and batch errors

[Widthdom]

Summary

• Reject unsupported MCP index arguments instead of accepting and ignoring them.
• Sanitize batch_query slot exceptions with the same tool-error sanitizer used by standalone tools/call.
• Preserve raw batch slot exception details in server stderr for local diagnostics.

Validation

• dotnet build -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --no-build --filter "FullyQualifiedName~ToolsCall_Index_RejectsUnsupportedArguments_Issue2848|FullyQualifiedName~ToolsCall_BatchQuery_SanitizesSlotExceptionMessage_Issue2849"
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• Adversarial review: found one logging-parity issue in the sanitized batch slot path, fixed it, and found no remaining blocking/actionable issues.

Documentation / Changelog

• changelog.d/unreleased/2848.fixed.md
• changelog.d/unreleased/2849.security.md
• No README/guide update was needed; the MCP schema already documented only the implemented index arguments and no public output fields changed.

Follow-up Candidates

None.

Fixes #2848
Fixes #2849