fix(ai_ckeditor): remove stale toolbar items on uninstall and warn on confirm page

Description

Fixes an issue where uninstalling ai_ckeditor left stale toolbar references in existing editor.editor.* configuration entities, causing CKEditor 5 text format configuration pages to break.

Two changes in ai_ckeditor.install:

• hook_module_preuninstall(): Before the module is removed, iterates over all editor.editor.* configs and strips the aickeditor and ai_balloon_menu toolbar items, as well as the ai_ckeditor_ai plugin configuration block.
• hook_form_FORM_ID_alter() for system_modules_uninstall_confirm_form: Reads the pending modules from the expirable key-value store and, if ai_ckeditor is being uninstalled, injects a "Editor configurations that will be updated" details block listing the affected editor.editor.* configs — inserted before the confirmation prompt so administrators are informed before they confirm.

Testing instructions

1. Install ai_ckeditor.
2. Go to Configuration → Content authoring → Text formats and editors and edit a CKEditor 5 text format (e.g. Full HTML).
3. Add the AI Balloon Menu button to the active toolbar and save.
4. Go to Extend → Uninstall and select AI CKEditor integration.
5. On the confirmation page, verify that an "Editor configurations that will be updated" section lists the affected editor.editor.*configs (e.g. editor.editor.full_html).
6. Confirm the uninstall.
7. Return to the text format configuration page — verify it loads correctly and no toolbar errors appear.
8. Export config (drush config:export) and verify no ai_balloon_menu or aickeditor references remain in any editor.editor.*.yml file.

Checklist

• I have linked the related issue in the MR title or
• I have performed a self-review of my own code
• I have added or updated tests, or explained in the description why this change is not covered by tests
• I have updated documentation for any new or changed functionality
• I have written testing instructions and verified them locally
• I have noted any required post-merge steps (config imports, cache rebuilds, manual changes)
• This MR contains no breaking API or hook changes, or they are explicitly documented in the description

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