Fine-tuning file uploads get server-side schema validation with processing_status

Files uploaded for fine-tuning now go through full server-side schema validation during ingestion, with the result exposed on the file object. Poll the Files API and read processing_status (COMPLETED, INVALID_FORMAT, or FAILED) plus validation_report to detect dataset issues programmatically before launching a job, like missing role fields or malformed conversation turns.

Errors include a user-facing reason, so you can fix the dataset and re-upload without trial-and-error training runs. For example:

Line 7: messages[1] must contain a role field