Epic: Switch WPT coverage report generation to use wpt-gen

## Background & Motivation We want to switch the WPT coverage report generation in ChromeStatus from directly calling the Gemini API to using the `wpt-gen` library. This will leverage the refactored, library-safe version of `wpt-gen`. ## Subtasks - [ ] #6345 Add wpt-gen dependency to requirements.txt - [ ] #6346 Integrate wpt-gen in GenerateWPTCoverageEvalReportHandler - [ ] #6347 Clean up legacy WPT coverage code in gemini_helpers.py _{This issue description was drafted by Gemini}