feat: #3631526 Move the video generator to an experimental, optional submodule

Issue: #3631526 "Initialize the AI Media module"

Chunk 11 of the UX and integration plan (20 October 2026): video, phase 1 (plan section 5), as decided by the maintainer: video moves to a submodule, off by default and experimental, until the AI module supports a motion prompt (#3586540).

Depends on !17 (closed) (3631526-generate-action), which depends on !16 (closed) to !8 (closed) and !6 (merged). Until those merge, this diff includes them; the change of this MR is its last commit.

What changed

• New submodule AI Media Video (modules/ai_media_video), lifecycle: experimental with lifecycle_link to #3586540, off by default. The Video generator plugin and the "Video generated with AI" media source (and its schema) move there; the main module no longer offers video.
• The generator attribute gains two optional properties: experimental (the Generators tab shows "Video (experimental)") and cost_note (the line under Generate in place of the paid request one). Video uses both: "Video generation is slow and usually costs much more than an image."
• "Not set up" for administrators now names only the operations that have no provider, and when no installed provider offers one, says so: "Video generation is not set up. No installed provider offers image_to_video. See Providers in the AI Media documentation for providers that do." Editors still read the plain sentence and get no tab.
• Providers tab: an operation row shows only when a generator on the site uses it (image to video only with the submodule), and says when no installed provider offers it. A stored choice for a row that is not shown is kept on save.
• Docs: the README and index tables mark Video as the experimental submodule; providers.md has a Video section with the provider facts from the plan (none of the AI module family offers image to video; Grok does, untested).
• The video keys stay in ai_media.settings (generators.video, editor_options.video, the example), so the defaults apply when the submodule is turned on; they are ignored without it. No update hook.
• Phase 2 (frame then motion, long jobs) is not started: it needs a prompt on ImageToVideoInput, an async contract in the AI module and a provider.

How it was tested

• Kernel VideoGeneratorTest (submodule): no video generator without the module, experimental with it; the admin message names image_to_video and not text_to_image; the video cost line. The consumer type test now expects no video consumer without the submodule. All 31 Kernel tests pass.
• Plain Drupal 11.4 (aimplan1) with the submodule and an AI video media type: the administrator sees the new message on /media/add/ai_video, the editor the plain sentence; Generators shows "Video (experimental)"; Providers shows the image to video row with the note.
• axe-core 4.10.2: 0 violations on the video "not set up" page, the Generators tab and the Providers tab, in Claro, Gin and Default Admin at 1440 and 1920.
• Varbase E2E free lane on a fresh CI-like site without the submodule: see the pipeline.
• phpcs (Drupal, DrupalPractice), phpstan level 1, cspell, eslint, stylelint: clean.

Not tested

• A real image-to-video provider (none in the AI module family; Grok not installed, no key).
• The video cost line in the browser (it shows only once video is set up, which needs such a provider; covered by the Kernel test).
• Mobile widths (out of scope).

AI-Generated: Yes. Written with Claude Code (Claude Opus 5.5) under the Drupal AI policy; tested by the author before submission.

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