test(workflow): cover the besluit variable type + start empty-body throw (refs #13)

Kills two mutation survivors: assert the complete besluit variable's type, and add
the missing start empty-response case (the pre-existing baseline survivor). Domain
mutation score reaches 100%.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

services/domain/Big.Tests/FlowableWorkflowClientTests.cs

@@ -128,6 +128,17 @@ public class FlowableWorkflowClientTests
             () => client.StartRegistrationProcessAsync(RegistrationId.New()));
     }
 
+    [Fact]
+    public async Task Start_throws_when_flowable_returns_an_empty_body()
+    {
+        var capture = new RequestCapture();
+        var client = Client(capture.Responds(HttpStatusCode.Created, "null"));
+
+        var ex = await Assert.ThrowsAsync<InvalidOperationException>(
+            () => client.StartRegistrationProcessAsync(RegistrationId.New()));
+        Assert.Contains("empty process-instance", ex.Message);
+    }
+
     [Fact]
     public async Task Complete_throws_when_flowable_rejects_the_request()
     {
@@ -227,6 +238,7 @@ public class FlowableWorkflowClientTests
             capture.Seen.RequestUri!.ToString());
         Assert.Contains("\"action\":\"complete\"", capture.Body);
         Assert.Contains("\"name\":\"besluit\"", capture.Body);
+        Assert.Contains("\"type\":\"string\"", capture.Body);
         Assert.Contains($"\"value\":\"{expected}\"", capture.Body);
     }