fix: convert str file path to Path before calling .stat() in _handle_file_or_url

[devin-ai-integration]

Summary

Fixes AttributeError: 'str' object has no attribute 'stat' when passing a string file path to client.document.generate(file="some_file.pdf").

The _handle_file_or_url method accepts Union[Path, str] for the file parameter, but the branch that handles file paths with extensions called file.stat() without first converting the string to a Path object. Added file = Path(file) before the .stat() call — this is safe for both str and Path inputs since Path(Path(...)) is a no-op.

Review & Testing Checklist for Human

• Verify fix by running client.document.generate(file="some_file.pdf", ...) with a string path — should no longer raise AttributeError
• Confirm passing a Path object still works as before

Notes

• The same Path(file) conversion was already done two lines below when calling self._client.files.upload(file=Path(file), ...), so this makes the code consistent.
• ImagePredictions._handle_images_or_urls is not affected — it uses a different pattern with str(image).

Link to Devin session: https://app.devin.ai/sessions/691f8a989ec340a397e56a2e178b91d7
Requested by: @DylanSoftware

[devin-ai-integration]

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