Generate is offered when the default provider module is not installed, then fails with "This could not be made"

Problem/Motivation

AI Media reads the default provider of each operation from ai.settings default_providers. When that provider's module is not installed (uninstalled later, or the config imported before the module), AiMediaGeneratorBase::defaultFor() still returns the default, so isConfigured() is TRUE: the editor is offered "Generate with AI", presses Generate, and gets "This could not be made. Try again." The paid-request line under the button shows the raw plugin id ("Each press is a paid request to openai.") because providerLabel() has no definition to read. The log carries PluginNotFoundException: The "openai" plugin does not exist.

Steps to reproduce

1. Set default_providers.text_to_image to a provider whose module is not enabled (or uninstall the provider module after setting it).
2. As a content editor, open Add media > Image: the Upload / Generate with AI switch is offered.
3. Describe an image and press Generate image: "This could not be made. Try again."

Proposed resolution

defaultFor() returns NULL when AiProviderPluginManager::hasDefinition() is FALSE for the configured provider id. The generator then counts as not set up: no switch for editors, and administrators read the "not set up" message naming the operation. Kernel test in ErrorMessagesTest.

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

• With a default provider whose module is missing, the Generate switch is not offered and administrators see the "not set up" message instead of a failed generation.

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: the switch is not offered and the plain upload form shows.

After, as an administrator: the not set up message names the operation and links to the AI settings.

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• A default provider whose module is not installed now counts as not set up instead of offering a Generate that fails.

AI-Generated: Yes