feat: add run_funnel_report tool using Data API v1alpha

[mderazon]

Summary

This PR picks up and completes the work from #54 (originally authored by @cagnusmarlsen), which added a run_funnel_report tool. That PR stalled due to merge conflicts and inactivity. This PR rebases cleanly on current main and incorporates all reviewer feedback from the original PR.

Changes

• analytics_mcp/tools/reporting/funnel.py (new file): run_funnel_report async function and its description builder _run_funnel_report_description()
• analytics_mcp/tools/reporting/metadata.py: adds get_funnel_steps_hints() with 7 proto-generated step examples covering common funnel configurations
• analytics_mcp/tools/utils.py: adds data_v1alpha import and create_data_api_alpha_client() factory function
• analytics_mcp/coordinator.py: registers run_funnel_report as an ADK FunctionTool alongside the existing tools
• README.md: documents the new tool

Differences from #54

• Adapted to current architecture: the original PR used an mcp.add_tool() pattern that no longer exists. This PR uses the current ADK FunctionTool registration in coordinator.py.
• Bug fix: the step_home_page_view example in get_funnel_steps_hints() now uses pageLocation with CONTAINS instead of pagePath with EXACT, which is not supported inside funnel steps (flagged by @testyuminliang in the original PR).
• Null safety: date_ranges parameter now safely handles None default (was crashing previously).
• Resolved merge conflicts: rebased cleanly on current main.

Tested this locally and it was working great