Instrument `generate_videos`

## Problem

`Models.generate_videos` (and async variant) is not instrumented. Video generation returns a `GenerateVideosOperation` (long-running operation) and supports text-to-video, image-to-video, frame interpolation, and video extension.

Growing in usage with Veo models.

## Files

* `sentry_sdk/integrations/google_genai/__init__.py` — `setup_once()` needs additional patches

## Acceptance Criteria

* `generate_videos` calls produce spans
* Async variant is also instrumented
* Model name and prompt are captured (prompt respects PII settings)
* Tests verify span creation