FFL-1294: Fix FFE system tests payload format

Fixed system tests to use correct unwrapped UFC payload format.
Remote Config delivers direct UFC payloads, not wrapped format.

Changes:
- Fixed flags-v1.json: Unwrapped UFC format (removed data.attributes wrapper)
- Fixed _load_ufc_fixture: Return direct JSON payload (addresses ruff lint)
- Fixed _set_and_wait_ffe_rc: Send unwrapped UFC data to RC

Author: leoromanovsky