fix: preserve the DeepSeek file object discriminator in upload results (#19406)

## Background

DeepSeek file uploads returned object: "file", but the published upload
result omitted that discriminator from providerMetadata.

## Root Cause

The DeepSeek response schema parsed the nullish object field, but
DeepSeekFiles.uploadFile did not map it into provider metadata; the
recorded response replay confirmed the resulting metadata loss.

## Summary

Conditionally mapped response.object to providerMetadata.deepseek.object
and added a patch changeset for @ai-sdk/deepseek.

## Testing

Updated file upload mapping coverage to assert object: "file" is
preserved and missing or null object values remain omitted.

## End-to-end Validation

- `pnpm -C packages/deepseek build && pnpm -C examples/ai-functions exec
tsx src/reproduction/issue-19390-deepseek-file-object-metadata.ts`
exited successfully and preserved `object: "file"`.
- `pnpm -C examples/ai-functions exec tsx
src/upload-file/deepseek/image.ts` successfully uploaded a live image,
exposed the discriminator, and completed generation.

## Related Issues

Fixes #19390

Closes #19394

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Co-authored-by: ai-sdk-factory <308175966+ai-sdk-factory@users.noreply.github.com>
Co-authored-by: gr2m <39992+gr2m@users.noreply.github.com>