feat(google): support batch APIs (#18470)

## Background

The experimental batch APIs currently support OpenAI and Anthropic.
Google’s Gemini Developer API also provides asynchronous batch
processing, but the Google provider did not implement the AI SDK batch
language model interface.

This PR adds Gemini Batch API support to `@ai-sdk/google`.

## Summary

- Implements the experimental text batch APIs for Google language
models.
- Uses inline batch requests when the creation payload is below 20 MB.
- Uploads larger inputs as JSONL through the Gemini Files API, up to
Google’s 2 GB limit.
- Supports inline and file-based batch results.
- Reuses the normal Google request preparation and response conversion
paths.