Creates a Gemini video extension job from an existing source video.
Unified extension request containing the source video and optional prompt/config overrides.
Optional_executionContext: MultiModalExecutionContextOptional multimodal execution context. Unused directly in this adapter.
Optionalsignal: AbortSignalOptional abort signal for request, polling, and download cancellation.
Normalized video artifacts plus provider metadata.
Gemini video extension capability implementation.
Uses Gemini's long-running video generation API with a source video payload, optionally polls until completion, and normalizes the extended video into
NormalizedVideo[].