Creates a Gemini video generation job and optionally waits for a terminal result.
Unified generation request containing prompt, optional reference image, and runtime params.
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 generation capability implementation.
Uses Gemini's long-running video generation API, optionally polls until the operation completes, and normalizes the generated video into
NormalizedVideo[].