Refactor test dependency resolution in DocSignedNotificationTests

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.

EnvelopeGenerator.Tests/TestBase.cs

@@ -24,6 +24,8 @@ public abstract class TestBase : Faker
 
     protected IRepository Repository => Host.Services.GetRequiredService<IRepository>();
 
+    protected IServiceProvider Services => Host.Services;
+
     protected abstract void ConfigureServices(IServiceCollection services);
 
     [SetUp]