Refactored DocSignedNotificationTests to use typed repository
retrieval and explicit service resolution for PsPdfKitAnnotation.
Added a Services property to TestBase for easier access to
IServiceProvider. These changes improve clarity and robustness
of test dependency management.
Changed test file namespaces from EnvelopeGenerator.Tests.Application to EnvelopeGenerator.Tests for consistency. Updated DigitalData.Core.Abstraction.Application package from 1.4.0 to 1.6.0 to incorporate latest improvements.
