fixup: pin CastAction and InheritanceCase BSON field keys

Address PR #365 ako review B1 and B2 by pinning the BSON shape Studio
Pro emits, rather than changing correct code based on a faulty premise.

A BSON dump of the Control Centre app (Mendix 9.24) shows Studio Pro
stores:

  Microflows$CastAction:
    VariableName = Account                # not OutputVariableName
    ErrorHandlingType = Rollback
  Microflows$InheritanceCase:
    Value = Administration.Account        # not Entity

The current writer matches both. The parser already falls back to the
alternative keys (OutputVariableName / Entity) for forward
compatibility, so projects authored by future Mendix versions that
ever rename these would still parse cleanly.

Add two regression tests so the writer cannot be "fixed" back to the
incorrect names without a CI failure:

  - TestSerializeCastAction_UsesVariableNameFieldKey
  - TestBuildSequenceFlowCase_InheritanceCase_UsesValueFieldKey

Each asserts both the presence of the correct key and the absence of
the alternative one, so a partial change is also caught.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hjothamendix

sdk/mpr/inheritance_roundtrip_test.go

@@ -63,3 +63,45 @@ func TestCastAction_RoundtripVariableName(t *testing.T) {
 		t.Fatalf("OutputVariable = %q, want SpecificInput", parsed.OutputVariable)
 	}
 }
+
+// TestSerializeCastAction_UsesVariableNameFieldKey pins the BSON field key
+// Studio Pro emits for Microflows$CastAction. Empirical evidence (BSON
+// dump of the Control Centre app on Mendix 9.24): Studio Pro stores the
+// output variable under "VariableName", not "OutputVariableName". The
+// parser falls back to "VariableName" when "OutputVariableName" is
+// absent so projects authored by Studio Pro still parse cleanly; the
+// writer must match Studio Pro's authored shape so projects we produce
+// open without surprises.
+func TestSerializeCastAction_UsesVariableNameFieldKey(t *testing.T) {
+	action := &microflows.CastAction{
+		BaseElement:    model.BaseElement{ID: "cast-1"},
+		OutputVariable: "SpecificInput",
+	}
+	doc := serializeMicroflowAction(action)
+	if got := bsonGetKey(doc, "VariableName"); got != "SpecificInput" {
+		t.Fatalf("VariableName = %v, want SpecificInput", got)
+	}
+	if got := bsonGetKey(doc, "OutputVariableName"); got != nil {
+		t.Fatalf("OutputVariableName = %v, want absent (Studio Pro uses VariableName)", got)
+	}
+}
+
+// TestBuildSequenceFlowCase_InheritanceCase_UsesValueFieldKey pins the
+// BSON field key for Microflows$InheritanceCase. Empirical evidence
+// (BSON dump of the Control Centre app, Mendix 9.24): the entity
+// reference is stored under "Value" as a qualified-name string
+// (e.g. "Administration.Account"), not "Entity". The parser falls back
+// to "Entity" for forward compatibility; the writer must emit "Value"
+// so output matches Studio Pro's authored shape.
+func TestBuildSequenceFlowCase_InheritanceCase_UsesValueFieldKey(t *testing.T) {
+	doc := buildSequenceFlowCase(&microflows.InheritanceCase{
+		BaseElement:         model.BaseElement{ID: "case-1"},
+		EntityQualifiedName: "Sample.SpecializedInput",
+	})
+	if got := bsonGetKey(doc, "Value"); got != "Sample.SpecializedInput" {
+		t.Fatalf("Value = %v, want Sample.SpecializedInput", got)
+	}
+	if got := bsonGetKey(doc, "Entity"); got != nil {
+		t.Fatalf("Entity = %v, want absent (Studio Pro uses Value)", got)
+	}
+}