Issue #3636953: Fix install crash on sites without node/taxonomy

Example and test SDC components referenced taxonomy_term and node via x-entity-type, so Canvas threw a PluginNotFoundException while calculating component config dependencies on sites where those modules aren't installed (the testing/minimal profiles).

Move all example/test components into a new hidden submodule, canvas_entity_reference_examples, that depends on node, taxonomy, media, and block_content. Also make EntityReferenceShapeMatcher check EntityTypeManagerInterface::hasDefinition() before matching an x-entity-type annotation, so any component referencing a not-installed entity type is treated as incompatible instead of crashing. Add a GenericModuleTestBase test to catch regressions.

AI-assisted: Claude was used to investigate this issue and draft the fix; reviewed and tested manually before submitting.

Issue: https://www.drupal.org/project/canvas_entity_reference/issues/3636953