Complete partial video generation foundation

[felixarntz]

While we partially had the required pieces for video generation already present, for some reason we were missing part of the foundation. This PR fixes that.

This does not actually implement any video generation features - it just completes the API. It will allow providers to implement video support in the future.

Nothing special here, this entirely follows the existing patterns we already use for e.g. text generation, image generation, speech generation, ...

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.54%. Comparing base (30a1506) to head (bda379c).
⚠️ Report is 7 commits behind head on trunk.

Additional details and impacted files

@@             Coverage Diff              @@
##              trunk     #217      +/-   ##
============================================
+ Coverage     87.45%   87.54%   +0.09%     
- Complexity     1194     1201       +7     
============================================
  Files            60       60              
  Lines          3858     3878      +20     
============================================
+ Hits           3374     3395      +21     
+ Misses          484      483       -1     

Flag	Coverage Δ
unit	87.54% <100.00%> (+0.09%)	⬆️

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[github-actions]

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Co-authored-by: felixarntz <flixos90@git.wordpress.org>
Co-authored-by: gziolo <gziolo@git.wordpress.org>

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[gziolo]

All checks pass locally (tests, PHPCS clean, PHPStan clean).

Verified the implementation follows the established patterns for speech/image generation:

• VideoGenerationModelInterface and VideoGenerationOperationModelInterface match the existing interface pairs (sync + async)
• PromptBuilder additions (generateVideoResult, generateVideo, generateVideos) follow the same flow: set output modality → delegate to generateResult with the correct capability
• Capability inference works from both the model interface and explicit output modality, consistent with other modalities
• The old "video not yet supported" throw is replaced with proper delegation, and the unsupported modality test now uses document instead
• Test coverage is thorough — mirrors the existing speech/image test structure across all code paths (result generation, error cases, shorthand helpers, capability inference, isSupported check)

Based on my research, the two-interface split (sync ModelInterface + async OperationModelInterface) is a good fit for providers like OpenAI/Sora where video generation is inherently asynchronous (job submission + polling), while the sync interface can wrap create_and_poll + download for simpler usage.

No issues found.