feat(api): specify delete entity-template behavior and enforce relation-target validation

[walterspieler-dkt]

What this PR Provides

• Adds a deletion guard for entity templates that are referenced as
  targetTemplateIdentifier by another template relation.
• Introduces a dedicated domain exception
  EntityTemplateIsRelationTargetException with a clear validation message.
• Implements cascade delete of all entities for a template before deleting the
  template itself, in the same transactional flow.
• Extends repository ports/adapters and JPA repositories to support:
  • relation-target usage check before deletion.
  • bulk deletion of entities by template identifier.
• Updates API behavior for DELETE /api/v1/entity-templates/{identifier}:
  • success response is now HTTP 200.
  • returns HTTP 400 when the template is still used as a relation target.
  • still returns HTTP 404 when template does not exist.
• Adds/updates tests to validate:
  • service-level cascade deletion call path.
  • controller-level HTTP 200 success behavior.
  • controller-level HTTP 400 when deletion is blocked by relation targeting.

ADR link:

• N/A

Fixes

• N/A

Review

The reviewer must double-check these points:

• The reviewer has tested the feature
• The reviewer has reviewed the implementation of the feature
• The documentation has been updated
• The feature implementation respects the Technical Doc / ADR previously produced
• The Pull Request title has a ! after the type/scope to identify the breaking
  change in the release note and ensure we will release a major version.

How to test

Please refer (copy/paste) the test section from the User Story. This should include

• The initial state: what should be the status of the system before testing
  • A running idp-core API with a seeded database containing:
    • an existing template not referenced as a relation target
      (example: monitoring-service).
    • a template referenced as relation target by at least one other template
      (example from current test data: microservice).
    • a non-existing template identifier for negative tests.
• What and how to test: steps to perform to test the feature
  • Call DELETE /api/v1/entity-templates/monitoring-service.
  • Verify the endpoint returns HTTP 200.
  • Verify entities belonging to monitoring-service are removed (for example,
    by checking related entity retrieval endpoints or DB content).
  • Call DELETE /api/v1/entity-templates/microservice.
  • Verify the endpoint returns HTTP 400 with an explanatory error message about
    relation-target usage.
  • Call DELETE /api/v1/entity-templates/<non-existent-id>.
  • Verify the endpoint returns HTTP 404.
• Expected results: what should be observed for success or failure
  • Successful deletion returns HTTP 200 and removes both template and owned
    entities.
  • Deletion is rejected with HTTP 400 when the template is still used as a
    relation target in another template.
  • Unknown template identifiers return HTTP 404.

Breaking changes (if any)

• API JSON schema modification (existing resource / behavior):
  • DELETE /api/v1/entity-templates/{identifier} success status changed from
    HTTP 204 to HTTP 200.
• Behavior modification of a component:
  • template deletion now explicitly fails with HTTP 400 when the template is a
    relation target for another template.
• Others:
  • cascade delete now explicitly removes all entities for the template before
    deleting the template.

Context of the Breaking Change

The delete-template endpoint behavior was made explicit and aligned with domain
integrity rules:

• prevent deletion when it would leave dangling relation definitions.
• ensure template-owned entities are purged in the same transactional flow.
• expose clear API outcomes for consumers (200/400/404).

Result of the Breaking Change

• API consumers that expected HTTP 204 on successful template deletion must
  adapt to HTTP 200.
• API consumers now receive HTTP 400 for relation-target constraint violations
  and should resolve referencing relations before retrying deletion.

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