Submit a direct file upload for a Multi FileFeed import

Submits (finalizes) a direct upload that was previously created via the direct-uploads endpoint. The upload_id is provided as a path parameter. The server verifies that the file was actually uploaded to the file store, updates the file size from storage metadata, and creates the record linking the file to this import.

The direct upload flow consists of three steps:

1. Create (direct-uploads): creates a tracking record and returns the presigned URL, content type, and upload ID.
2. Upload: the client uploads the file directly to cloud storage using the presigned URL.
3. Submit (this endpoint): verifies the upload succeeded and links the file to the import.