Story 4.4: Test Core Type Definitions and API Protocols

As a developer, I want pytest tests for shared core type definitions, So that Pydantic models, protocol constants, and type validation are verified as a foundation for other OPEA tests.

FRs: FR20 | Sprint: 22

Acceptance Criteria:

• tests/test_core.py verifies custom Pydantic models serialize/deserialize correctly
• Tests verify protocol constants match expected values
• Tests verify type validation on request/response models
• This story is a prerequisite for Stories 4.5 and 4.6 (other OPEA tests depend on these types)