Add tests

Problem/Motivation

The subentity module lacks automated test coverage for several critical classes. ReferencedEntityAccessControlHandler has partial kernel test coverage but several paths are untested. EntityParentHandler::getStorageByEntityType() and the Drush generator (SubEntityGenerator) have no tests at all.

Proposed resolution

tests/src/Kernel/EntityParentHandlerTest.php

Kernel tests covering EntityParentHandler::getStorageByEntityType():

• Returns empty array when no referencing field exists
• Groups results by parent entity type
• Handles multiple referencing fields on the same parent type
• Excludes fields targeting other entity types

tests/src/Unit/SubEntityGeneratorTest.php

Unit tests covering the Drush generator:

• All templates referenced in generate() exist on disk
• routing.yml.twig uses the module-specific administer subentities permission and not the generic administer site configuration

tests/src/Kernel/ReferencedEntityAccessControlHandlerTest.php

Additional cases for the existing test class:

• checkAccess() when no parent references the subentity → should return neutral()
• checkAccess() when the parent exists but denies the operation → should return neutral()
• checkCreateAccess() when a referencing field exists and the parent allows creation → should return allowed()

Remaining tasks

• Create tests/src/Kernel/EntityParentHandlerTest.php
• Create tests/src/Unit/SubEntityGeneratorTest.php
• Update tests/src/Kernel/ReferencedEntityAccessControlHandlerTest.php
• Verify all tests pass via phpunit --group subentity