Use Drupal AI's default providers, and remove AI Media's Providers tab

Problem/Motivation

Drupal AI manages the AI providers: keys, models and the default provider and model for each operation type, on its own settings page. AI Media keeps a second copy of that choice in its Providers tab (providers in ai_media.settings). That is extra code and settings to keep in step with Drupal AI, and a site has to set the same thing in two places.

Steps to reproduce

1. Set the default Text to image provider in Drupal AI's settings.
2. Open the AI Media settings: its Providers tab has its own Text to image choice.

Proposed resolution

• Always use Drupal AI's default provider and model for each operation type (Text to image, Image to video, Chat, Chat with image vision).
• Remove the Providers tab and the providers settings, schema and defaults.
• Where a default is missing, say so and link to Drupal AI's settings page.
• Update the Kernel tests and the docs.

AI-Generated: Yes

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

• The Providers tab is removed from the AI Media settings.

API changes

• None.

Data model changes

• providers is removed from ai_media.settings.

Release notes snippet

• AI Media uses the default AI providers set in Drupal AI.