Supersedes #555

Linked to 1st Phase of #584

1. AasRepositoryBackend is an interface for which Spring automatically generates a Bean based on the available CrudRepositories.
2. AasRepositoryBackend extends the interface AasServiceOperations. Spring injects AasServiceOperations as a Repository Fragment, aligning with the recommended approach for achieving native Spring composition.
3. AasServiceOperations serves as a reusable interface for both AasRepositoryBackend and AasService:
   • In AasRepositoryBackend, it is provided by Spring as a composable Fragment.
   • In AasService, it is injected as a dependency.
4. By default, Spring searches for an implementation named AasServiceOperationsImpl within the same package as the interface. However, this approach does not align with the current architecture, which distributes backend implementations across different packages and modules.
   1. To address this, AasRepositoryFragmentConfiguration is introduced to manage custom injection of AasServiceOperations. If a Bean named aasServiceOperations is available, it is automatically composed into AasRepositoryBackend. This ensures that the location of the implementation no longer affects its integration.
   2. For example, MongoDbAasServiceOperations (the MongoDB implementation of AasServiceOperations) can reside in the aasservice-backend-mongodb package while being utilized by aasrepository-backend-mongodb by registering a Bean with the expected naming convention. This approach enhances flexibility and control over the overall backend implementation.
5. Another key design decision was to make AasServiceOperations function more like a @service. Specifically:
   • Exceptions are thrown in a manner consistent with typical service-layer behavior.
   • This contrasts with Spring's CrudRepository, where certain operations—such as failing to find or delete a non-existent entity—are silently ignored.
   • This approach optimizes backend-specific exception handling when AasServiceOperations is used within CrudAasRepository (not to be confused with Spring’s CrudRepository).
   • As a result, there is no need to pre-check an entity’s existence at the service layer before calling AasServiceOperations—the appropriate exception is thrown immediately with minimal database queries.