feat(workflow): add durable video generation (#19627)

## Summary

- add an experimental `@ai-sdk/workflow/video` video generation helper
that starts a job with a durable Workflow webhook
- **suspend the workflow for the entire video render, consuming no
compute until the webhook resumes it**
- run start and status operations in steps, using a stable step-derived
idempotency key for billable starts
- return raw provider `url | base64 | binary` video data without
downloading hosted videos across step boundaries
- add API docs, README usage, unit coverage, a Workflow suspend/resume
integration test, and a patch changeset

## Example

```ts
import { experimental_generateVideo as generateVideo } from '@ai-sdk/workflow/video';

export async function videoWorkflow(prompt: string) {
  'use workflow';

  // The workflow suspends while the video renders, consuming no compute.
  const result = await generateVideo({
    model: 'klingai/kling-v3.0-t2v',
    prompt,
  });

  return result.videos;
}
```

## Suspension behavior

While `generateVideo()` waits for the provider to render the video, the
workflow is durably suspended. No function remains running and no
compute is consumed. The provider webhook resumes the workflow, which
then makes one status request to retrieve the result.