Use non-deprecated DAL result access in mail validation

[Copilot]

The highest-version Actions check failed because Shopware 6.8 deprecates EntitySearchResult::first().

• Compatibility: Retrieve the mail-template type through getEntities()->first().
• Regression coverage: Verify the resolved technical name is passed to the mail finder.

$mailTemplateType = $repository->search($criteria, $context)->getEntities()->first();