Add integration tests (@QuarkusTest) for REST API endpoints

[pcastelo]

Problem

No integration tests verify the full REST API contract end-to-end with a running Quarkus instance. Current tests are mostly unit-level with mocked dependencies.

Acceptance criteria

• Add @QuarkusTest integration tests for critical endpoints:
  • POST /api/migration/analyze — full analyze flow with fixture data
  • POST /api/migration/apply — apply with mock K8s client
  • GET /api/threescale/products — product listing
  • POST /api/cluster/targets — cluster registration
  • POST /api/chat — chat endpoint with mock AI
• Use @QuarkusTestResource for external dependency stubs
• Test error responses (400, 404, 500) and input validation
• Add contract tests: verify OpenAPI spec matches actual responses
• Integration tests run in CI as part of pr-check.yml