Issue #3586455: Add kernel tests for requirements hook, entity item cache...

Description

Adds kernel test coverage for four areas of the ai_context module that had no automated tests in 1.0.x:

1. ai_context_requirements() hook (AiContextRequirementsTest) — verifies that non-runtime phases return empty, that a site with the bundled ai_context_editorial workflow passes, and that a site with no content-moderation workflow covering ai_context_item returns REQUIREMENT_WARNING.
2. AiContextEntityItemCacheContext (AiContextEntityItemCacheContextTest) — verifies getContext() returns 'none' with no entity, returns 'entity_type:id' when an entity is set, getCacheableMetadata() includes the correct cache tag, and that an unknown entity type is swallowed gracefully rather than crashing the cache system.
3. AiContextOverviewSettingService (AiContextOverviewSettingServiceTest) — verifies saveShowOverviewPage() persists the value to config, that invalidateIfChanged() is a no-op when the value is unchanged, and that it triggers a menu cache clear and route rebuild when the value changes without leaving the router in a broken state.
4. ai_context_post_update_0001_remove_canvas_page_from_target_entity_types() (AiContextCanvasPagePostUpdateTest) — verifies canvas_page is removed while other types are preserved, the function is idempotent when canvas_page is absent, returns early when the config object does not exist, and that the resulting array is re-indexed.

All 16 tests pass with no failures. The 21 deprecation notices are pre-existing from the ai module dependency and are not introduced by this MR.

Testing Instructions

Clone the module and check out this branch inside an existing Drupal 11 + ai_context development environment:

cd web/modules/contrib/ai_context git fetch ai_context-3586455 git checkout 3586455-add-automated-ccc-tests-for-remaining-10-coverage-gaps

Run the new kernel tests from the Drupal root:

phpunit -c web/core web/modules/contrib/ai_context/tests/src/Kernel/AiContextRequirementsTest.php phpunit -c web/core web/modules/contrib/ai_context/tests/src/Kernel/AiContextEntityItemCacheContextTest.php phpunit -c web/core web/modules/contrib/ai_context/tests/src/Kernel/AiContextOverviewSettingServiceTest.php phpunit -c web/core web/modules/contrib/ai_context/tests/src/Kernel/AiContextCanvasPagePostUpdateTest.php

Expected result: Tests: 16, Assertions: 125 — all pass. Deprecation notices (from the ai module) are pre-existing and not introduced by this MR.

Checklist

• I have updated the MR title to use format: Issue #1234: My issue title.
• I have updated the MR description to include: Closes #1234
• 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

AI Compliance

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Closes #3586455