feat(ai): expose individual image generation `calls` (#19959)

## Background

`generateImage()` can split one request into multiple provider calls,
but its existing aggregate result does not preserve the association
between an image and the call that produced it. This makes per-call
response metadata, costs, warnings, and usage difficult to consume
reliably. This change addresses #19938.

## Summary

- Add `result.calls`, preserving images, response metadata, provider
metadata, warnings, and usage for each underlying provider call.
- Add optional `providerMetadata` to `GeneratedFile`
- Core projects legacy `providerMetadata.<provider>.images[index]` onto
each generated image.
- Keep top-level aggregate `warnings` and `usage`, deprecate `responses`
and merged top-level `providerMetadata`.