Describe the bug
In an event with a moderated registration form, rejecting a registration that has already been accepted does not update the registration status or send the rejection email. The UI displays a success message, but the registration remains in the Complete state and no notification is issued. The behavior suggests either a logic gap or an unclear design choice.

To Reproduce

1. Open an event with a moderated registration form.
2. Accept a pending registration.
3. In the registration list, select that same user.
4. From the Moderation menu, click Reject Registration.
5. Observe that a success message appears, but the registration status remains Complete and no rejection email is sent.

Expected behavior
Rejecting a registration—regardless of its prior accepted state—should update its status to Rejected and trigger the configured rejection email containing the provided reason. If this is not intended due to workflow semantics, the UI should prevent the action or display a warning rather than a success message.

Screenshots

Additional context
Given that registration.state != RegistrationState.pending is the first exit condition in _modify_registration_status, it seems like this was a conscious design choice. However the RH should at least count how many registrations were skipped if that's the case.

A workaround is possible: resetting the registration first and then rejecting it correctly updates the status and sends the rejection email.