test(domain): a behandelaar can take a registration into behandeling (refs #13)

First step of S-12a (beoordeling decision model): an INGEDIEND registration moves
to IN_BEHANDELING when a behandelaar picks it up. Red — TakeIntoBehandeling() and
the InBehandeling status do not exist yet.

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

services/domain/Big.Tests/RegistrationTests.cs

@@ -121,4 +121,14 @@ public class RegistrationTests
         Assert.Contains("only an INGEDIEND", ex.Message);
         Assert.Equal(RegistrationStatus.Ingeschreven, registration.Status);
     }
+
+    [Fact]
+    public void Taking_a_registration_into_behandeling_moves_it_to_in_behandeling()
+    {
+        var registration = Registration.Submit("123456782");
+
+        registration.TakeIntoBehandeling();
+
+        Assert.Equal(RegistrationStatus.InBehandeling, registration.Status);
+    }
 }