Fix namespace formatting in BaseEmailInterface::attach() PHPDoc

Problem/Motivation

The PHPDoc type hint in BaseEmailInterface::attach() method is incorrectly formatted as Drupal\symfony_mailer\AttachmentInterface instead of the proper fully qualified class name format \Drupal\symfony_mailer\AttachmentInterface.

This missing leading backslash can cause issues with IDE autocompletion, static analysis tools, and code documentation generators that expect proper namespace formatting.

Steps to reproduce

Notice the type hint @param Drupal\symfony_mailer\AttachmentInterface $attachment is missing the leading backslash.

Proposed resolution

Add the missing leading backslash to the PHPDoc type hint to make it a proper fully qualified class name: @param \Drupal\symfony_mailer\AttachmentInterface $attachment.

Remaining tasks

• Create patch to fix the PHPDoc type hint
• Review and test the patch
• Commit the fix

User interface changes

None.

API changes

None.

Data model changes

None.