Adds batch API support for image and video generation.

- `image.prepare()` / `video.prepare()` to create batch requests
- `input_file_id` param on `batch.create()` for JSONL file uploads
- Typed `image_response` / `video_response` properties on batch results
- Deduplicates request building via `_make_generate_request` in image
module

Fix unknown status handling in polling loops
- Replace ValueError with warnings.warn for unknown statuses in video
and collections polling
- Continue polling instead of crashing when encountering unrecognized
statuses
- Add tests for unknown status warning behavior in sync and async video
clients
- Update warning messages to be more descriptive

Improve TimeoutError messages with operation context
- Add optional  parameter to  for descriptive timeout errors
- Pass context strings from all polling callers (collections, video,
chat)
- Round timeout duration to 1 decimal place in error message
- Update timeout test assertion to check for contextual message

- Add `Union[ImageGenerationModel, str]` to all image client methods
(sample, sample_batch, prepare)
- Add `Union[VideoGenerationModel, str]` to all video client methods
(generate, start, prepare)
- Enables IDE autocomplete for model names, matching existing ChatModel
pattern

Prepare to release `v1.9.0` of the xAI Python SDK which includes
- Support for image and video generation in batch requests (#112)
- Support for using JSONL files in batch requests (#112)
- Bug fix that prevents crashing on unknown statuses for polling in long
running requests such as video generation and document indexing (#113)
- Use of model literals for video and image models for IDE autocomplete
(#114)