Replaced GetOrThrow methods with a fluent GetOr/Throw pattern for handling null or empty MediatR responses. Introduced GetOrContext<TResponse> struct with Throw, ThrowNotFound, ThrowInvalidOperation, and ThrowBadRequest methods. Updated all tests to use the new API and added coverage for new exception types. Improved XML docs and performed minor code cleanup.
Renamed SendOrThrowAsync and SendOrNotFoundAsync extension methods for IMediator to GetOrThrow for ISender, following MediatR best practices. Updated all usages, XML docs, and tests to use ISender and the new method names. Replaced StubMediator with StubSender in tests. Functionality remains the same, but code now aligns with modern MediatR conventions.
Introduce MediatorExtensionsTests to cover SendOrThrowAsync and SendOrNotFoundAsync extension methods for IMediator, including edge cases and cancellation. Refactor CreateEnvelopeCommand in Fake.cs to use Authorize(userId) instead of setting UserId directly. Add test stubs for IMediator and IRequest to support isolated testing.
