[13.x] Populate mailable recipients in mail notifications

[LucasCavalheri]

Summary

When a notification toMail() method returns a Mailable instance without recipients, the MailChannel currently sends the message without automatically using the notifiable mail route.

This differs from the existing MailMessage behavior, where recipients are automatically resolved from the notifiable.

This PR aligns both behaviors by automatically populating recipients for Mailable notifications only when no recipients were explicitly defined.

Changes

• Populate recipients from the notifiable mail route when:

  • the returned Mailable has no to recipients
  • and the Mailable envelope() does not define recipients

• Preserve existing behavior when:

  • recipients were explicitly defined using ->to()
  • recipients were defined via envelope()->to

• Remove the unused $message parameter from getRecipients()

Motivation

MailMessage notifications already resolve recipients automatically from the notifiable.

Returning a Mailable from toMail() should behave consistently and avoid requiring users to manually duplicate recipient resolution logic for the most common notification flow.

This also improves developer experience and reduces subtle delivery mistakes when converting notifications from MailMessage to Mailable.

Backward Compatibility

This change is fully backward compatible.

Existing applications that already define recipients explicitly on the Mailable are unaffected because the channel only populates recipients when none are present.

Tests

Added tests/Notifications/MailChannelTest.php covering:

• populates recipients from the notifiable
• does not override explicit ->to()
• respects envelope()->to
• supports multiple addresses returned by routeNotificationForMail()

Additional integration coverage was added to SendingNotificationsWithLocaleTest.php.