test(backend): type-state Aanvraag builder, illegal fixtures unrepresentable (WP-70)

RuleTests/OpenZaakZaakSourceTests hand-built Aanvraag fixtures by initializer,
keeping Submitted/Referentie/SubmittedAt/BesluitStatus consistent by hand. A
type-state builder (Given.Concept().Submitted().Decided()) makes an illegal
sequence a compile error instead, and delegates the toelichting-required rule
to the real BeoordelingRules so it can't drift from production. Adds
BesluitLifecycleTests covering the WP-68 besluit invariants end to end.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/tests/BigRegister.Tests/RuleTests.cs

@@ -1,10 +1,10 @@
-using BigRegister.Api.Data;
 using BigRegister.Domain.Applications;
 using BigRegister.Domain.Beoordeling;
 using BigRegister.Domain.Diplomas;
 using BigRegister.Domain.Documents;
 using BigRegister.Domain.Registrations;
 using BigRegister.Domain.Submissions;
+using BigRegister.Tests.Builders;
 
 namespace BigRegister.Tests;
 
@@ -207,35 +207,24 @@ public class BeoordelingRuleTests
   // StatusAt, and CanDecide refuses a further besluit regardless of which one. Pins the
   // domain statement "Afgewezen/Goedgekeurd → no further besluit" independent of the
   // endpoint's own (integration-level) Already_decided_case_rejects_a_further_besluit.
-  private static Aanvraag Decided(Besluit besluit) => new()
-  {
-    Id = "x",
-    Type = "registratie",
-    Owner = "test",
-    Submitted = true,
-    Referentie = "BIG-2026-1",
-    SubmittedAt = DateTimeOffset.UtcNow,
-    CreatedAt = DateTimeOffset.UtcNow,
-    UpdatedAt = DateTimeOffset.UtcNow,
-    BesluitStatus = besluit,
-    BesluitToelichting = besluit == Besluit.Goedkeuren ? null : "toelichting",
-  };
-
+  // WP-70: built via Given, not a hand-rolled Aanvraag literal — Decided(Besluit.Afwijzen) with
+  // no toelichting simply couldn't compile as a fixture here.
   [Theory]
   [InlineData(Besluit.Goedkeuren)]
   [InlineData(Besluit.Afwijzen)]
   public void A_terminal_decision_refuses_any_further_besluit(Besluit recorded)
   {
     var now = DateTimeOffset.UtcNow;
-    var tag = Decided(recorded).StatusAt(now).Tag!.Value;
-    Assert.False(BeoordelingRules.CanDecide(tag));
+    var toelichting = recorded == Besluit.Goedkeuren ? null : "toelichting";
+    var aanvraag = Given.Concept(owner: "test").Submitted().Decided(recorded, toelichting).Build();
+    Assert.False(BeoordelingRules.CanDecide(aanvraag.StatusAt(now).Tag!.Value));
   }
 
   [Fact]
   public void MeerInfoOpvragen_is_not_terminal_a_further_besluit_is_still_legal()
   {
     var now = DateTimeOffset.UtcNow;
-    var tag = Decided(Besluit.MeerInfoOpvragen).StatusAt(now).Tag!.Value;
-    Assert.True(BeoordelingRules.CanDecide(tag));
+    var aanvraag = Given.Concept(owner: "test").Submitted().Decided(Besluit.MeerInfoOpvragen, "toelichting").Build();
+    Assert.True(BeoordelingRules.CanDecide(aanvraag.StatusAt(now).Tag!.Value));
   }
 }