Implement FastAPI CRUD endpoints from skeletons

Given a FastAPI project with two working endpoint functions (create and read) and shared Pydantic models, implement the remaining update and delete endpoints to complete CRUD. Requirements: a) match the existing request/response schemas and status codes; b) preserve idempotency and return 404 vs 204 appropriately; c) validate inputs and handle exceptions; d) write unit tests for all four endpoints; e) explain how you would approach this if you have never used FastAPI before, by leveraging the existing endpoints as templates and the framework documentation.