[PP-1] Refactor GeneratedFieldExplicitInputUxComponentSourceBase and FieldForComponentSuggester to need only SDC's ComponentMetadata, not SDC plugin instances

Made a start on this.

I don't think this needs to be a stable blocker; XB at release won't yet allow additional ComponentSourceInterface implementations. Which means this would be an entirely internal refactor?

The fact we had to pretend this exists in the fallback plugin makes me feel it is a stable blocker.
I know @mohit_aghera has a pinto component source plugin that is also grappling with this.

Adding back tag until the conversation above is resolved (we can always remove it again, just want to make sure it's not lost)

I think this is less urgent.

Tagging for colleagues as this is a bug bear for the Pinto work Mohit has been doing

Thanks, @mohit_aghera, for pushing this forward! 🥳🙏

I read #10, but I don't think this is a stable blocker. We'd be happy to merge this once it's ready though, so by untagging it I have no desire to slow this issue down.

For XB, the definition of stable is different than core's. We'll be adding @internal to almost everything and relaxing that in 1.x minor releases. The main goal of an XB stable release is to unblock a Drupal CMS 2.0 stable release, allowing people/agencies to build and deploy real sites with that. But XB 1.0.0 won't yet have a proper PHP API surface for contrib projects to perform more advanced integrations than what DCMS 2 exercises, except to the extent that issues like this one just happen to get done ahead of a 1.0.0 release.

This is going to block 🌱 [META] Content templates Active , and 📌 Move `PropSourceEndpointTest` into new `XbConfigEntityHttpApiTest::testComponent()` Active made it very apparent 😅

Quoting \Drupal\Tests\experience_builder\Functional\ApiUiContentTemplateControllersTest::testSuggestionsClientErrors():

   *           ["node/article/js.xb_test_code_components_with_link_prop", 400, "Code components are not supported yet."]
   *           ["node/article/js.xb_test_code_components_with_no_props", 400, "Code components are not supported yet."]

So, starting point:

diff --git a/src/Controller/ApiUiContentTemplateControllers.php b/src/Controller/ApiUiContentTemplateControllers.php
index 85bcf22f9..3f3909582 100644
--- a/src/Controller/ApiUiContentTemplateControllers.php
+++ b/src/Controller/ApiUiContentTemplateControllers.php
@@ -112,11 +112,6 @@ final class ApiUiContentTemplateControllers extends ApiControllerBase {
       throw new BadRequestHttpException('Only components that define their inputs using JSON Schema and use fields to populate their inputs are currently supported.');
     }
 
-    // @todo Add support for suggestions for code components in https://www.drupal.org/i/3503038
-    if (!$source instanceof SingleDirectoryComponent) {
-      throw new BadRequestHttpException('Code components are not supported yet.');
-    }
-
     if ($this->entityTypeManager->getDefinition($content_entity_type_id, FALSE) === NULL) {
       throw new NotFoundHttpException(sprintf("The `%s` content entity type does not exist.", $content_entity_type_id));
     }