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)

agent/component/fillup.py

@@ -64,11 +64,14 @@ class UserFillUp(ComponentBase):
         for k, v in kwargs.get("inputs", {}).items():
             if self.check_if_canceled("UserFillUp processing"):
                 return
-            if isinstance(v, dict) and v.get("type", "").lower().find("file") >=0:
+            if isinstance(v, dict) and v.get("type", "").lower().find("file") >= 0:
                 if v.get("optional") and v.get("value", None) is None:
                     v = None
                 else:
-                    v = FileService.get_files([v["value"]])
+                    file_value = v["value"]
+                    # Support both single file (backward compatibility) and multiple files
+                    files = file_value if isinstance(file_value, list) else [file_value]
+                    v = FileService.get_files(files)
             else:
                 v = v.get("value")
             self.set_output(k, v)