Improvements

UnitPrice of OrderLine shouldn't be part of the API call, it should be looked up in DB based on ProductName (or even better, an id).

Separate domain models from API. Right now we need to consider the shape of the API when updating the models for the services.

Document all response types and status codes in the OpenAPI specification.

CancellationTokens are never a bad idea.

Use an injected TimeProvider. This will help reduce flakey unit tests as the tests no longer rely on actual wall time.

Non of the OrderServiceTests are very interesting, because there's no interesting logic in the OrderService. There's more interesting logic in RefundService, some of which is still untested.

Consider moving the test project to /test. If not I'd suggest to simply remove /src and have all projects in the root directory.

Add tests for the OrdersController. Now both GetById and RefundOrder has logic, other than mapping, that could be tested. It would also be good to test the validation.

Consider upgrading to .NET 10. No harm in being on the latest .NET version, especially when it's an LTS version. For a project of this size, and age, it should be a no-brainer. For bigger/older projects, it might be easier to upgrade now and take a few breaking changes, than it is to upgrade to in the future (to the then current (LTS) version) with even more breaking changes.

Dependencies should be looked over and upgraded where possible.

Task 4: Considerations

Using the built-in ProblemDetails instead of a custom exception format for a unified response format on all kinds of errors.

My philosophy is that any exception thrown should be truly exceptional, and therefore all exceptions should be returned as 500 errors. They should of course be logged and an alert should be sent to the team to investigate. In case of an expected error happening in the service (or lower) it should be visible in the return type, and the controller should map it to an appropriate response. Right now the only expected "error" is not finding an order, so returning a nullable order is fine, but if other expected errors (such as unauthorized access maybe) is added to the service logic then the return type has to change.