Remove MediatorGetOrContext; add ExecuteAsync overloads

Removed MediatorGetOrContext.cs, eliminating the fluent API for handling null or empty MediatR responses with custom exceptions. Added two ExecuteAsync overloads to FinalizeDocumentJob: one for processing a single EnvelopeDto and another for processing all envelopes with the EnvelopeCompletelySigned status.

EnvelopeGenerator.ServiceHost/Jobs/FinalizeDocumentJob.cs

@@ -60,6 +60,8 @@ public class FinalizeDocumentJob(IOptions<WorkerOptions> options, ILogger<Finali
         }
     }
 
+    public Task ExecuteAsync(EnvelopeDto envelope, CancellationToken cancel = default) => ExecuteAsync([envelope], cancel);
+
     public async Task ExecuteAsync(CancellationToken cancel = default)
     {
         var envelopes = await mediator.Send(new ReadEnvelopeQuery()