Installing the module fails on sites without the taxonomy module: "The "taxonomy_term" entity type does not exist

Originating from https://git.drupalcode.org/project/drowl_canvas/-/work_items/3615406
Summarized with the help of AI:

Problem/Motivation

Installing canvas_entity_reference on a site without the Taxonomy module (and likewise Node) fails. The module only depends on canvas:canvas, but it ships example and test SDCs that reference taxonomy_term and node via x-entity-type. When Canvas generates the Component config entities for these SDCs during the install, it calculates their dependencies. That fails because the referenced entity type does not exist:

Drupal\Component\Plugin\Exception\PluginNotFoundException: The "taxonomy_term" entity type does not exist.

web/core/lib/Drupal/Core/Entity/EntityTypeManager.php:171
web/modules/contrib/canvas/src/PropExpressions/StructuredData/FieldPropExpression.php:139
web/modules/contrib/canvas/src/PropExpressions/StructuredData/ReferenceFieldTypePropExpression.php:217
web/modules/contrib/canvas/src/PropSource/StaticPropSource.php:639
web/modules/contrib/canvas/src/Plugin/Canvas/ComponentSource/JsonSchemaPropsComponentSourceBase.php:181
web/modules/contrib/canvas/src/Plugin/Canvas/ComponentSource/SingleDirectoryComponent.php:120
web/core/lib/Drupal/Core/Plugin/PluginDependencyTrait.php:71
...
web/modules/contrib/canvas/src/Entity/Component.php:194
web/modules/contrib/canvas/src/ComponentSource/ComponentSourceManager.php:238
web/modules/contrib/canvas/src/ComponentSource/ComponentSourceManager.php:124
web/modules/contrib/canvas/src/Hook/ComponentSourceHooks.php:95
web/core/lib/Drupal/Core/Extension/ModuleInstaller.php:257

The failing component is sdc.canvas_entity_reference.example-taxonomy-card (prop category: items.x-entity-type: taxonomy_term). Other bundled components reference taxonomy_term or node as well: test-multi-nodes, test-bundle-filter, test-node-ref, test-node-id.

This also breaks GenericModuleTestBase tests of every module that depends on canvas_entity_reference, because those run on the testing profile without Taxonomy and Node.

Steps to reproduce

1. Use a site without the Taxonomy module, e.g. the testing or minimal install profile.
2. Install canvas_entity_reference: drush en canvas_entity_reference
3. The install fails with the exception above.

Alternatively, a generic module test reproduces it:

namespace Drupal\Tests\canvas_entity_reference\Functional;

use Drupal\Tests\system\Functional\Module\GenericModuleTestBase;

/**
 * @group canvas_entity_reference
 */
class GenericCanvasEntityReferenceTest extends GenericModuleTestBase {

  protected function assertHookHelp(string $module): void {}

}

Tested with Drupal core 11.4.11, Canvas 1.12.0, Canvas Entity Reference 1.0.4.

Proposed resolution

• Move the example and test SDCs (group "Entity Reference Tests") into a test module (e.g. tests/modules/canvas_entity_reference_test), or into an optional submodule that depends on drupal:node and drupal:taxonomy. They should not be in the component library of production sites anyway.
• Make the prop shape matching more robust: in EntityReferenceHooks::storablePropShapeAlter() / EntityReferenceShapeMatcher, don't map props whose x-entity-type does not exist (EntityTypeManagerInterface::hasDefinition()). Canvas then flags the component as incompatible instead of crashing. Note: in a downstream module, marking such props as unstorable in a later hook_canvas_storable_prop_shape_alter() was not enough to avoid the crash for example-taxonomy-card, so this may need further investigation.
• Add a GenericModuleTestBase test to catch this.

Remaining tasks

• Decide between a test module and an optional submodule for the example components
• Patch / MR
• Tests

User interface changes

The example / test components no longer show up in the Canvas component library on production sites.

API changes

None.

Data model changes

None.