IBX-12339: Log a warning when notifier subscription is missing

[vidarl]

🎫 Issue	IBX-12339

Related PRs:

• IBX-12339: Email sending not working : Invite user recipes-dev#257

Strictly speaking not related, but needed in order to make the invite user functionality work end-to-end (IBX-12340):

• IBX-12340: Pass the required struct option when registering from invitation user#140

Description:

Ibexa\AdminUi\Notifier\UserInvitation::sendInvitation() and
Ibexa\AdminUi\Notifier\PasswordReset::sendMessage() both return without sending and without any
trace when notifications.subscriptions does not carry their notification class:

if ($this->isNotifierConfigured()) {
    $this->sendNotification($invitation);
}

Both classes are LoggerAware and are handed a logger, but nothing is logged if the notification
class is not mentioned in notifications.subscriptions.

This PR turns the silent return into a logged warning, naming the exact configuration key to add.

For QA:

1. Remove Ibexa\AdminUi\Notifier\Notification\UserInvitation from
   ibexa.system.<scope>.notifier.subscriptions, clear cache, and invite a user from the back
   office. The invitation row is still written to ibexa_user_invitation; expect no mail, and in
   var/log/dev.log:
   app.WARNING: No invitation e-mail was sent: subscribe Ibexa\AdminUi\Notifier\Notification\UserInvitation under "ibexa.system.<scope>.notifier.subscriptions" to enable it.
2. Add the subscription back, clear cache, invite again: mail is sent and nothing is logged.
3. Same two steps for password reset via Forgot password and
   Ibexa\AdminUi\Notifier\Notification\UserPasswordReset.

Documentation:

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud