Check the generate permission before filtering the media add list

Problem/Motivation

Without the generate media with ai permission, /admin/content/media/generate answers 403, but /media/add?ai=1 typed by hand still lists AI types with ?ai_media=1 links. They fall back to upload, so nothing is exposed, but the list is misleading.

Steps to reproduce

1. Log in as a user who can add media but lacks generate media with ai.
2. Open /media/add?ai=1.

Proposed resolution

Filter the list only when the user has the permission.

Remaining tasks

• ✅ File an issue
• ❌ Addition/Change/Update/Fix
• ❌ Testing to ensure no regression
• ➖ Automated unit/functional testing coverage
• ➖ Developer Documentation support
• ➖ User Guide Documentation support
• ➖ 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

User interface changes

• Before (1.0.x): without the permission, the list is filtered and links to Generate mode.
  
• After: core's Add media list with plain links.
  

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• N/A

AI-Generated: Yes