Convert the procedural hooks to OOP hook classes in the Varbase Email module

Problem/Motivation

Drupal 11 introduced object-oriented hook implementations via the #[Hook] attribute (change record). Converting the module's procedural hooks to an OOP hook class improves structure and aligns with Drupal core. An MR is opened per branch as needed.

Steps to reproduce

Proposed resolution

Move hook_theme() and hook_preprocess_email() from varbase_email.module into src/Hook/VarbaseEmailHooks.php with #[Hook] attributes. No #[LegacyHook] wrapper (Drupal ~11.4-only).

Remaining tasks

• ✅ File an issue about this project
• ✅ Addition/Change/Update/Fix to this project
• ✅ Testing to ensure no regression
• ➖ Automated unit/functional testing coverage
• ➖ Developer Documentation support on feature change/addition
• ➖ User Guide Documentation support on feature change/addition
• ➖ UX/UI designer responsibilities
• ➖ Accessibility and Readability
• ✅ Reviewed by a human
• ✅ Code review by maintainers
• ✅ Full testing and approval
• ✅ Credit contributors
• ✅ Review with the product owner
• ✅ Update Release Notes
• ✅ Release varbase-9.2.0, varbase_email-9.2.0

User interface changes

• N/A

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• task: #3608321 Remove the #[LegacyHook] shims for full OOP hooks