Add session video dimensions options

[bc-anam]

Summary

• add video_width and video_height to SessionOptions
• serialize them as sessionOptions.videoWidth and sessionOptions.videoHeight
• validate that dimensions are positive integers and provided together

Verification

• uv run --extra dev pytest

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Summary by cubic

Add support for optional session video dimensions. You can now set video_width and video_height in SessionOptions to request a specific output size.

• New Features
  • Add video_width and video_height; must be provided together and be positive integers.
  • Serialize as videoWidth and videoHeight in the session payload.
  • Update README with a supported-resolutions table (Cara-3/Cara-4, landscape and Cara-4 portrait examples) and add tests for serialization and validation.

^{Written for commit 36f7cd9. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 3 files

_{Re-trigger cubic}