Users can send a private message to themselves by typing their own name into the recipient field

The recipient autocomplete correctly hides the current user from its suggestions, so a sender cannot pick themselves from the list. However, this filter only works on the suggestion level. If the sender manually types their own name with a user ID (e.g. "John Doe (123)"), the value passes validation and the message is sent, creating a thread addressed to the sender alone. The same happens when a role consisting only of the sender is entered as a recipient. Since the autocomplete already prevents self-selection, the validation should reject the sender as a recipient as well.