feat: enable multi-file upload for chat and agent workflows (#12977)

### Closes: #12921 

### What problem does this PR solve?

Previously, multi-file upload was not working correctly across the
application:

- **Chat**: UI displayed "Upload max 5 files" but only the first file
was actually uploaded
- **Agent conversational mode**: Frontend sent multiple files but
backend only processed one
- **Agent task-mode file inputs**: Explicitly limited to single file
only

This PR enables proper multi-file upload support for both chat and agent
workflows, allowing users to upload and process multiple files (up to 5)
as the UI originally suggested.

**Changes:**
- `web/src/pages/next-chats/hooks/use-upload-file.ts`: Process all files
instead of only `files[0]`
- `api/apps/canvas_app.py`: Handle multiple files via
`files.getlist("file")`
- `web/src/pages/agent/debug-content/uploader.tsx`: Allow up to 5 files
with `multiple={true}`
- `agent/component/begin.py` & `fillup.py`: Support file arrays while
maintaining backward compatibility

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

api/apps/canvas_app.py

@@ -253,11 +253,14 @@ async def upload(canvas_id):
 
     user_id = cvs["user_id"]
     files = await request.files
-    file = files['file'] if files and files.get("file") else None
+    file_objs = files.getlist("file") if files and files.get("file") else []
     try:
-        return get_json_result(data=FileService.upload_info(user_id, file, request.args.get("url")))
+        if len(file_objs) == 1:
+            return get_json_result(data=FileService.upload_info(user_id, file_objs[0], request.args.get("url")))
+        results = [FileService.upload_info(user_id, f) for f in file_objs]
+        return get_json_result(data=results)
     except Exception as e:
-        return  server_error_response(e)
+        return server_error_response(e)
 
 
 @manager.route('/input_form', methods=['GET'])  # noqa: F821