feat(ai-grok): video generation adapter for the grok-imagine video models

[tombeckenham]

Problem

xAI's Imagine API ships video generation models, but @tanstack/ai-grok has no video adapter — generateVideo() currently supports OpenAI Sora and fal only.

Available models (per docs.x.ai/developers/models):

• grok-imagine-video — text-to-video, $0.05/second
• grok-imagine-video-1.5-preview — text-to-video (preview), $0.08/second

Proposal

Add a grokVideo adapter following the existing jobs/polling pattern (BaseVideoAdapter, like the OpenAI Sora and fal video adapters):

• createVideoJob / getVideoStatus / getVideoUrl against the Imagine video endpoints
• imageInputs support for image-to-video / starting-frame flows, reusing the metadata.role conventions from image-to-image and image-to-video support #618 (start_frame, etc.)
• Aspect-ratio sizing consistent with the grok-imagine image models added in feat(ai): add imageInputs / videoInputs / audioInputs for image-conditioned generation #624 (size: '16:9_2k' template → aspect_ratio / resolution)

Notes

• The Imagine API plumbing landed in feat(ai): add imageInputs / videoInputs / audioInputs for image-conditioned generation #624 for the image models (grok-imagine-image, grok-imagine-image-quality): JSON endpoints called directly (the OpenAI SDK's multipart paths are rejected by xAI), aspect-ratio size template, and /v1/images/edits image conditioning. The video adapter can build on the same utilities.
• E2E: aimock doesn't mock the Imagine video endpoints, so coverage would follow the same unit-test approach as the image-to-image / image-to-video features (see testing/e2e/src/lib/feature-support.ts).

Related: #618, #624.