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.
All test files and utilities now use the EnvelopeGenerator.Tests.Application namespace for improved organization and clarity. No functional changes were made; updates are limited to namespaces and using directives. This makes it explicit that these are application-level tests and related helpers.
