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.
Generalize null/empty response handling with SendOrThrowAsync<TResponse, TException>, allowing custom exceptions via a factory delegate. SendOrNotFoundAsync now wraps this method for NotFoundException. Improves type safety, flexibility, and XML docs; avoids treating strings as collections.
Introduced MediatorExtensions with SendOrNotFoundAsync methods to enforce non-null and non-empty responses from MediatR requests. These extensions throw NotFoundException when responses are null or empty, centralizing not-found logic and improving error handling.
