fix: use file existence check instead of extension heuristic in _handle_file_or_url

[devin-ai-integration]

Summary

The previous logic in _handle_file_or_url used file extension presence (Path(file).suffix) to decide whether a string was a local file path or a file_id. This broke when a file_id contained an extension (e.g. "document-abc123.pdf") — the SDK would attempt to upload a non-existent local file instead of passing it through as a file_id.

The new logic checks file.exists() on disk instead:

• If the file exists on disk → upload it
• Otherwise → treat it as a file_id

Also removes the assert not Path(file).suffix guard that prevented file_ids with extensions from being used at all.

Review & Testing Checklist for Human

• Verify behavior when a Path object points to a non-existent file: Previously this would attempt upload and fail with a clear FileNotFoundError from files.upload(). Now it silently treats it as a file_id, which may produce a confusing API error downstream. Consider whether a Path object (as opposed to a str) should always be treated as a local file and raise early if missing.
• Test with an actual local file path passed as a string (e.g. file="./my_doc.pdf" where the file exists) to confirm uploads still work correctly.
• Test with a file_id that contains an extension (e.g. file="document-abc123.pdf") to confirm it's passed through as a file_id without attempting upload.

Notes

• The else: raise ValueError(...) branch is now only reachable for non-Path, non-str inputs, which matches the type signature Optional[Union[Path, str]].
• files.upload() already has its own file.exists() check (line 122 in files.py), so the double-check is redundant for the upload path but harmless.

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

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment and CI monitoring