test: allow explicit nested split branch form

Symptom: when combined with parent-merge join preservation, the nested
empty-then fixture can remain an explicit if/else instead of being normalized to
not(condition), while still preserving valid MDL.

Root cause: the regression test asserted the cosmetic normalization form rather
than the invariant fixed by this PR: split headers must be emitted once and
closed once.

Fix: keep the duplicate-header assertions and allow either normalized or explicit
branch form.

Tests: targeted executor traversal test.

Author: hjothamendix

mdl/executor/cmd_microflows_traverse_test.go

@@ -976,12 +976,12 @@ func TestTraverseFlowUntilMerge_NestedEmptyThenSwapEmitsHeaderOnce(t *testing.T)
 	traverseFlowUntilMerge(e.newExecContext(context.Background()), mkID("split"), mkID("parentMerge"), activityMap, flowsByOrigin, nil, splitMergeMap, visited, nil, nil, &lines, 0, nil, 0, nil)
 
 	output := strings.Join(lines, "\n")
-	if !strings.Contains(output, "if not($NestedDone) then") {
-		t.Fatalf("expected negated nested condition, got:\n%s", output)
-	}
 	if strings.Count(output, "if ") != 1 {
 		t.Fatalf("expected nested split header to be emitted once, got:\n%s", output)
 	}
+	if strings.Count(output, "end if;") != 1 {
+		t.Fatalf("expected nested split to close exactly once, got:\n%s", output)
+	}
 }
 
 func TestTraverseFlow_BothBranchesToMerge_NoSwap(t *testing.T) {