ENH-007 — Test Coverage for Error Scenarios 🟠

[hajk1]

The test suite covers only the happy path. Manual API testing reveals unhandled edge cases: missing
entities return 500 instead of 404, business rule violations surface as unstructured errors, and
lazy associations accessed outside a transaction throw 500s. This issue adds negative-path coverage
and fixes the underlying service-layer issues that these tests expose.

Use FreightOrderControllerTest as the reference pattern for test setup.

404 — Entity not found (one test per controller):

• VesselControllerTest — GET /vessels/9999 → 404
• VoyageControllerTest — GET /voyages/9999 → 404
• FreightOrderControllerTest — GET /freight-orders/9999 → 404
• PortControllerTest — GET /ports/9999 → 404
• CustomerControllerTest — GET /customers/9999 → 404
• AgentControllerTest — GET /agents/9999 → 404
• InvoiceControllerTest — invoice for non-existent order ID → 404

400 — Validation failures:

• VoyageControllerTest — create voyage with missing required fields → 400
• FreightOrderControllerTest — create order with missing voyageId, containerId, or
  customerId → 400
• FreightOrderControllerTest — discountPercent > 100 → 400

Business rule violations:

• FreightOrderControllerTest — book the same container on two active voyages → 400
  (double-booking)
• InvoiceControllerTest — generate invoice for an order not in DELIVERED status → appropriate
  4xx (not 500)

Underlying fixes required by these tests:

• Any service method that throws IllegalArgumentException for a not-found case must throw
  ResponseStatusException(NOT_FOUND) instead — the GlobalExceptionHandler then returns 404
• Any service method that accesses a lazy association must be annotated @Transactional or
  @Transactional(readOnly = true) as appropriate

Acceptance criteria:

• All 404 tests listed above implemented and passing
• All 400 validation tests implemented and passing
• Business rule violation tests implemented and passing
• All services that return not-found throw ResponseStatusException(NOT_FOUND), not 500
• All service methods accessing lazy associations are @Transactional
• No existing test regresses
• Code is formatted

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