Get Task Agent Run File

Download a file associated with a TaskAgentRun. Files include uploads submitted with the run and files generated by Task tools during the run. The file is served with appropriate content type headers for inline viewing or download based on the `inline` parameter. `file_name` is the `stored_filename` from the run's `files` array, not the name the caller uploaded. Each entry there also carries a ready-made `download_url` for this endpoint, and a `source` of `input` or `output` saying which side of the run produced it. The response carries the stored file's own content type — a `.txt` attachment comes back as `text/plain`, not as an opaque byte stream — and a `Content-Disposition` header naming the stored file.