fix: #3634875 Use whole translatable strings per generator instead of putting the type label into a sentence

Issue: https://www.drupal.org/project/ai_media/issues/3634875

Each generator now declares its own complete strings as whole t() strings: field title, Generate button, progress message, mode switch label, and the "not offered" and "not set up" messages. AiMediaGeneration::noun() and the @what label placeholders are removed. Translators get whole sentences instead of a type label put into English, which languages with gender, case or articles cannot translate.

English output is unchanged. Generators from other modules get generic whole-sentence fallbacks. A Kernel test pins every string and shows an Arabic translation reaching the button and the switch.

Before (1.0.x): "Décrivez le image", the article cannot follow the noun. Varbase Starter, Gin, French, Image and SVG pages.

After: "Décrivez l'image" and "Générer une image" as whole French strings. Varbase Starter, Gin, French, Image and SVG pages.

AI-Generated: Yes

Checkpoints:

• File an issue
• Addition/Change/Update/Fix
• Testing to ensure no regression
• Automated unit testing coverage
• Automated functional testing coverage
• UX/UI designer responsibilities
• Readability
• Accessibility
• Performance
• Security
• Developer Documentation
• User Guide Documentation
• Reviewed by human
• Code review by maintainers
• Full testing and approval
• Credit contributors
• Review with the product owner
• Release notes snippet
• Release

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