feat: add run_pivot_report tool for pivot table reports

[ZLeventer]

Summary

• Adds a new run_pivot_report MCP tool that wraps the GA4 Data API's runPivotReport endpoint
• Pivot reports reorganize data by rotating dimensions into columns, similar to pivot tables in spreadsheets — useful for cross-tabulating metrics across two or more dimensions (e.g. sessions by country x browser) in a single API call
• Includes comprehensive get_pivot_hints() in metadata.py with example pivot structures showing field_names, limit, offset, and order_bys
• Follows the same patterns as run_report and run_realtime_report (description override, schema required fields, snake_case protobuf field names)

Changes

• analytics_mcp/tools/reporting/pivot.py — New tool module with run_pivot_report function and _run_pivot_report_description() hint builder
• analytics_mcp/tools/reporting/metadata.py — Added get_pivot_hints() function with 4 example pivot configurations
• analytics_mcp/coordinator.py — Registered the new tool with FunctionTool, description override, and required schema fields

Test plan

• Verify pivot.py, metadata.py, and coordinator.py pass syntax checks
• Run nox -s tests* to confirm no regressions
• Test with a live GA4 property by running a pivot report (e.g. sessions by country x browser)
• Verify the tool appears in list_tools with correct schema and required fields