feat(mcp): add source coverage status and search diagnostics

[Razpines]

Summary

• add source-type coverage reporting to status() (�vailable_source_types, source_type_counts, coverage_warnings)
• add search(debug=true) response envelope with structured debug metadata
• validate search(source_types=...) and return actionable error payloads for unsupported/unavailable source types
• add tests for source-type validation, debug mode, and status coverage metadata

Issues

• Closes feat(status/setup): expose source coverage counts and guard against missing Script Reference indexing #27
• Closes feat(dx): improve MCP search/status ergonomics (available_source_types, source_types validation, debug output) #31

Testing

• python -m py_compile src/unity_docs_mcp/mcp_server.py src/unity_docs_mcp/tools/ops.py tests/test_mcp_meta.py
• $env:PYTHONPATH='C:\projects\UnityRAG\src'; python -m pytest tests/test_mcp_meta.py -q