[13.x] Add attachFromStorage helpers to notification MailMessage

[LucasCavalheri]

Summary

This PR adds attachFromStorage and attachFromStorageDisk helpers to notification MailMessage.

These methods mirror the storage attachment API available on mailables and delegate to Attachment::fromStorageDisk() before attaching the generated attachment to the message.

Motivation

After #58201, mailables support attaching files directly from storage using dedicated helpers.

Notification mail messages can already attach Attachment instances, but they do not provide the same convenience methods for storage-backed attachments. This creates a small API inconsistency between Mailable and notification MailMessage.

Adding these helpers improves parity and makes notification mail attachments more ergonomic.

Changes

• Added attachFromStorage() to MailMessage
• Added attachFromStorageDisk() to MailMessage
• Delegates storage attachment creation to Attachment::fromStorageDisk()
• Applies custom attachment name and MIME options when provided
• Added test coverage for local and disk-specific storage attachments

Example

MailMessage::make()
    ->attachFromStorage('invoices/1.pdf')
    ->attachFromStorageDisk('s3', 'invoices/1.pdf', 'invoice.pdf', [
        'mime' => 'application/pdf',
    ]);

Tests

Added coverage in NotificationMailMessageTest:

• testItAttachesFilesFromStorage
• testItAttachesFilesFromStorageDisk

Verified with:

./vendor/bin/phpunit tests/Notifications/NotificationMailMessageTest.php
./vendor/bin/pint

Backward Compatibility

This change is fully backward compatible.

It only adds new convenience methods to notification MailMessage and does not alter existing attachment behavior.