Show error message while submitting when user doesn't participate in a challenge.

Error message like `you have not participated in the challenge` is not shown when a user tries to submit to a challenge without participating in it.

Reproduce a submission attempt for a challenge the user has not joined and inspect the CLI path that handles the response. Confirm that the expected participation error is displayed, then verify the behavior with the relevant submission checks or manual command output.

Written by the indexing model from the issue text.