fix: treat terminal nested IF as returning in flow builder

lastStmtIsReturn only matched a literal ast.ReturnStmt as the last
statement, missing the case where a body ends in an IF whose branches
both return. The outer IF then thought its THEN branch continued,
emitted a duplicate "true" SequenceFlow plus an orphan EndEvent with
no ReturnValue, and Studio Pro rejected the file with "Sequence
contains no matching element" on any diff or microflow open.

Widen the predicate to "last stmt is a guaranteed terminator" —
RETURN, RAISE ERROR, or an IF/ELSE where every branch is terminal
(recursively). LOOP is explicitly not terminal because BREAK exits
even if the body returns.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: 2 people

mdl/executor/cmd_microflows_builder_flows.go

@@ -172,11 +172,37 @@ func newUpwardFlow(originID, destinationID model.ID) *microflows.SequenceFlow {
 	}
 }
 
-// lastStmtIsReturn checks if the last statement in a body is a RETURN statement.
+// lastStmtIsReturn reports whether execution of a body is guaranteed to terminate
+// (via RETURN or RAISE ERROR) on every path — i.e. control can never fall off the
+// end of the body into the parent flow.
+//
+// Terminal statements: ReturnStmt, RaiseErrorStmt. An IfStmt is terminal iff it
+// has an ELSE and both branches are terminal (recursively). A LoopStmt is never
+// terminal — BREAK can exit the loop even if the body returns.
+//
+// Naming kept for history; the predicate is really "last stmt is a guaranteed
+// terminator". Missing this case causes the outer IF to emit a dangling
+// continuation flow (duplicate "true" edge + orphan EndEvent), which Studio Pro
+// rejects as "Sequence contains no matching element" when diffing.
 func lastStmtIsReturn(stmts []ast.MicroflowStatement) bool {
 	if len(stmts) == 0 {
 		return false
 	}
-	_, ok := stmts[len(stmts)-1].(*ast.ReturnStmt)
-	return ok
+	return isTerminalStmt(stmts[len(stmts)-1])
+}
+
+func isTerminalStmt(stmt ast.MicroflowStatement) bool {
+	switch s := stmt.(type) {
+	case *ast.ReturnStmt:
+		return true
+	case *ast.RaiseErrorStmt:
+		return true
+	case *ast.IfStmt:
+		if len(s.ElseBody) == 0 {
+			return false
+		}
+		return lastStmtIsReturn(s.ThenBody) && lastStmtIsReturn(s.ElseBody)
+	default:
+		return false
+	}
 }