Fix GAIA attachment downloads

## Summary

The current GAIA dataset provides attachment `file_path` values as repository-relative paths, but the Space currently checks them directly as local filesystem paths.

This change uses `hf_hub_download()` to resolve repository-relative attachment paths to files in the local Hugging Face cache. Existing absolute local paths continue to use the previous behavior.

## Validation

Tested on a private duplicate of the current Space:

- The application loaded all 20 questions successfully.
- All 5 referenced attachments were downloaded and mapped.
- `/files/{task_id}` returned HTTP 200 for:
- PNG
- MP3 (2 files)
- Python
- XLSX

The change is intentionally limited to attachment path resolution; unrelated loading and API behavior are unchanged.

## Related reports

- https://github.com/huggingface/agents-course/issues/624
- https://github.com/huggingface/agents-course/issues/647
- https://huggingface.co/proxy/discuss.huggingface.co/t/attachements-not-available-on-https-agents-course-unit4-scoring-hf-space-docs/171219

main.py

@@ -107,15 +107,22 @@ def load_questions():
 
             # 3. Store the file path mapping if file details exist and are valid
             if local_file_path and file_name:
-                 # Log if the path from the dataset isn't absolute (might indicate issues)
                  if not os.path.isabs(local_file_path):
-                     logger.warning(f"Task {task_id}: Path '{local_file_path}' from dataset is not absolute. This might cause issues finding the file on the server.")
-                    
-
-                 if os.path.exists(local_file_path) and os.path.isfile(local_file_path):
+                     try:
+                         local_file_path = hf_hub_download(
+                             repo_id="gaia-benchmark/GAIA",
+                             repo_type="dataset",
+                             filename=local_file_path,
+                         )
+                         logger.info(f"Downloaded file for task_id {task_id} to: {local_file_path}")
+                     except Exception as e:
+                         logger.warning(f"Could not download file '{local_file_path}' for task_id {task_id}: {e}")
+                         local_file_path = None
+
+                 if local_file_path and os.path.exists(local_file_path) and os.path.isfile(local_file_path):
                      task_file_paths[str(task_id)] = local_file_path
                      logger.debug(f"Stored file path mapping for task_id {task_id}: {local_file_path}")
-                 else:
+                 elif local_file_path:
                      logger.warning(f"File path '{local_file_path}' for task_id {task_id} does NOT exist or is not a file on server. Mapping skipped.")
             elif task_id: