Add comprehensive unit test coverage across domain and service layers

[devin-ai-integration]

Summary

Adds ~66 unit tests across 13 new test files covering the previously untested domain model and service layers:

• ftgo-domain (10 test files): Order (state machine lifecycle + invalid transitions), OrderLineItem, OrderLineItems, Restaurant, MenuItem, RestaurantMenu, Consumer, Courier, Plan, Action
• ftgo-order-service (1 test file): OrderServiceTest — order creation, cancellation, revision, acceptance with courier assignment, and full state transition chain (APPROVED → DELIVERED), with mocked repositories
• ftgo-consumer-service (1 test file): ConsumerServiceTest — validation, creation, lookup
• ftgo-courier-service (1 test file): CourierServiceTest — creation, availability toggling, lookup

Build dependency changes:

• Added junit:junit:4.12 and mockito-core:2.+ to ftgo-domain (had no test deps)
• Added mockito-core:2.+ to ftgo-consumer-service
• Added mockito-core:3.12.4 + byte-buddy:1.12.10 to ftgo-order-service and ftgo-courier-service to work around Mockito 2.x / Java 17 incompatibility when mocking Spring Data repository interfaces

Review & Testing Checklist for Human

• Mockito version inconsistency: ftgo-consumer-service uses mockito-core:2.+ (floating) while ftgo-order-service and ftgo-courier-service pin mockito-core:3.12.4 + byte-buddy:1.12.10. The split exists because consumer-service avoids mocking CrudRepository directly (uses reflection instead). Verify this is acceptable or if all modules should be aligned.
• Reflection-based mock injection in ConsumerServiceTest (lines 26-29): Uses Field.setAccessible(true) to inject the mock ConsumerRepository because ConsumerService uses @Autowired field injection. This is fragile — will break silently if the field is renamed. Consider whether this is acceptable or if ConsumerService should be refactored to constructor injection.
• shouldAcceptOrder mocks the Courier class (OrderServiceTest line 126) because Courier.plan is not initialized outside JPA context, so addAction() NPEs on a real instance. This means the courier delivery planning logic is not fully exercised. Verify the assertion (verify(courier, times(2)).addAction(...)) is sufficient.
• Run ./gradlew :ftgo-domain:test :ftgo-common:test :ftgo-consumer-service:test :ftgo-courier-service:test :ftgo-order-service:test locally to confirm all new tests pass on your environment.

Notes

• The explicit byte-buddy:1.12.10 dependency is needed because the version bundled with Spring Boot 2.0.3's Mockito 2.15.0 uses sun.misc.Unsafe.defineClass which was removed in Java 17.
• Pre-existing OrderControllerTest failures (unrelated to this PR) were not modified.

Link to Devin session: https://app.devin.ai/sessions/3e95da912de843389401182e2d4515bc
Requested by: @tobydrinkall

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✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

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