feat(llm): add generate_video tool for Google Veo

[williamjameshandley]

Summary

Adds a generate_video MCP tool to the llm server so Google Veo models become callable.
Today list_models() advertises veo-3.1-generate-preview / veo-2.0-generate-001 (defined in
gemini/models.yaml), but the only generation tool exposed is generate_image — there is no
binding that invokes the video API. This wires that up, mirroring generate_image's provider
routing and adapter dispatch.

What's included

• video_generation_adapter (gemini): async generate_videos + operations.get polling with
  a post-loop done check, files.download fallback when video_bytes is unpopulated, and
  image-to-video via types.Image (imported as GenAIImage to avoid colliding with the existing
  from PIL import Image).
• Registry: new "video_generation" adapter type; video_generation capability flag
  (pricing_type == "per_second") added to both capability builders.
• Model loader / YAML: default_duration_seconds threaded from YAML through
  build_model_configs_dict for per-second pricing (fail-fast — no magic number in code).
• generate_video tool: capability-gated dispatch, set-only kwarg forwarding,
  expanduser file write, per-second cost via seconds_generated, metadata-only return
  (no inline preview — video is written to output_file).
• Tests: unit tests for the adapter (config construction, polling, download mutation,
  timeout/error/RAI paths, image resolution, duration threading, capability flag) and
  MCP-protocol integration tests for the tool.
• README + version bump (0.31.11b10 → 0.32.0).

Design notes

• generate_audio is intentionally not exposed: the Gemini Developer API rejects the parameter
  (Enterprise-only), and Veo 3.1 emits native audio by default.
• Scoped to Gemini Veo. Grok video models (grok-imagine-video*) are real entries in
  grok/models.yaml and show video_generation: true, but use a separate SDK — a Grok video
  adapter is deferred to a follow-up; calling generate_video with a Grok model fails loud via
  get_adapter.

Testing

• 718 passed, ruff check/ruff format clean.
• Live end-to-end verified against the real Veo 3.1 API: an 8s 720p clip with an AAC audio
  track was generated, polled, downloaded, and written. The generate_audio-rejection above was
  caught by this live run — the mocked tests had passed straight through it.
• Integration tests mock the SDK at the get_client boundary by design (a real VCR cassette would
  require credentials and commit a multi-MB video binary).

🤖 Generated with Claude Code