feat: add check_compatibility tool

[ZLeventer]

Summary

• Adds a new check_compatibility MCP tool that wraps the GA4 Data API checkCompatibility method
• Allows users to verify which dimensions and metrics can be combined in a report before running it
• Accepts property_id, optional dimensions/metrics lists, and optional dimension_filter/metric_filter

Closes #61

Changes

• analytics_mcp/tools/reporting/compatibility.py — new async check_compatibility function following the same patterns as run_report
• analytics_mcp/coordinator.py — registers check_compatibility as a FunctionTool in the MCP tool list
• tests/compatibility_test.py — unit tests verifying the function is importable and async

Test plan

• nox -s tests* passes (new + existing tests)
• Manual test via Gemini or MCP client: call check_compatibility with a property ID and a set of dimensions/metrics, verify the response lists compatible/incompatible fields