Issue #3586515: Port FWA/Automators generalization from !1536 to 2.x

Summary

Ports the field widget action (FWA) / Automators generalization from !1536 (merged) (issue #3577050 (closed), merged to 1.4.x) onto 2.x, as requested in #3586515. The 1.x work could not be cherry-picked directly across the major-version boundary, so this is a 3-way port of the same change set.

What changed

The per-widget automator dispatch logic is generalized into AutomatorBaseAction:

• runAutomatorSubmit() runs the automator once during the submit phase and setFormInput() writes results into $form_state user input, so the rebuilt form scaffolds enough delta slots for every value returned.
• Two extension points let plugins customize only what they need:
  • transformFormInput() — remap a field item to the widget's expected input shape (e.g. target_id → fids).
  • setFormInput() — override for widgets that declare multiple_values = TRUE (single element for all deltas, e.g. boolean checkbox, options_select, tags autocomplete).
• updateItemsCount() bumps the widget's items_count before rebuild so extra values aren't dropped.
• Single-value list widgets share a new ListBase parent (ListString/ListInteger/ListFloat are now thin subclasses).
• Most per-widget plugins shed their bespoke aiAutomatorsAjax() / saveFormValues() boilerplate. Plugins with non-standard form trees keep custom dispatch: Chart, ImageAltText, ImageFilename, TextToImageMediaLibrary, TextToAudioMediaLibrary, Metatag, OfficeHours, SummaryTextareaWithSummary, ClassificationOptionsSelect.

Net: 29 files, +1224 / −1614 (boilerplate removed, logic centralized).

Automated tests

Adds FieldWidgetActionFormInputTest (kernel, 16 tests / 261 assertions) covering each input shape: default per-delta, flat multi-value, fids remap, scalar list input, sub-key (state), summary-only merge, legacy no-op plugins, and items_count bumping/non-shrinking.

# from project root (DDEV)
ddev exec bash -lc 'SIMPLETEST_DB="mysql://db:db@db/db" \
  vendor/bin/phpunit -c web/core \
  web/modules/contrib/ai/modules/ai_automators/tests/src/Kernel/Plugin/FieldWidgetAction/FieldWidgetActionFormInputTest.php'

The existing AutoCompleteTagsTaxonomyTest (FunctionalJavascript) continues to cover the tags-autocomplete flow end to end on CI.

Manual testing

1. Enable AI Automators and a provider; create an Automator for a target field.
2. Edit the field's form-display widget → enable the matching Field Widget Action and select the Automator.
3. On the node add/edit form, click the automator button on that field and confirm:
   • Single-value widgets (text, email, telephone, list select/radios, boolean, link, number) populate the generated value into the widget.
   • Multi-value fields receive one value per delta, and the widget grows to hold all returned values (no values silently dropped).
   • Tags autocomplete populates comma-separated tags; options_select / chosen receive a flat list; boolean checkbox toggles correctly.
   • Remap cases: image/file widgets show the previewed file (target_id → fids); moderation state sets via the state sub-key; summary+textarea updates only the summary while preserving the body you typed.
   • Special layouts (Chart, alt-text on media library, text-to-image/audio media library, metatag, office hours) still populate as before.
4. Confirm the AJAX rebuild leaves no console/PHP errors and the values persist on save.

Notes for reviewers

• Verified locally: kernel suite 16/16 green, phpcs (Drupal, DrupalPractice) clean.
• phpstan (level 7, non-blocking in CI) reports the inherited missingType.iterableValue nags on the new/moved methods — same category accepted in !1536 (merged); no new logic-level findings.
• A 2.0.x backport will follow in a separate MR once this lands.