### 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)
### What problem does this PR solve?
Fillup component return value not object
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
### What problem does this PR solve?
Add mechanism to check cancellation in Agent.
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
### What problem does this PR solve?
#10056
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
- [x] New Feature (non-breaking change which adds functionality)
### What problem does this PR solve?
#9082#6365
<u> **WARNING: it's not compatible with the older version of `Agent`
module, which means that `Agent` from older versions can not work
anymore.**</u>
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
