fix: #3634660 Count a default provider whose module is not installed as not set up
Closes https://www.drupal.org/project/ai_media/issues/3634660
A default provider in ai.settings whose module is not installed (uninstalled later, or config imported before the module) made isConfigured() TRUE: editors were offered Generate with AI and got "This could not be made. Try again.", and the paid-request line showed the raw plugin id. defaultFor() now returns NULL when the provider plugin has no definition, so the generator reads as not set up: no switch for editors, the "not set up" message for administrators.
Kernel test: ErrorMessagesTest::testMissingProviderModuleIsNotSetUp.
Before: the editor is offered Generate with AI, presses Generate and gets "This could not be made. Try again."; the paid-request line names the raw plugin id.
After, as a content editor: no switch, the plain upload form.
After, as an administrator: the not set up message names the operation and links to the AI settings.
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
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- Reviewed by human
- Code review by maintainers
- Full testing and approval
- Credit contributors
- Review with the product owner
- Release notes snippet
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