test(workflow): erp-workflow 单元测试从 16 增至 63 — 覆盖 model/error/parser/expression/executor

- model.rs: 13 个 FlowGraph 测试（build/outgoing/incoming/start-end 边界）
- error.rs: 7 个 WorkflowError → AppError 转换测试
- parser.rs: 11 个新增验证边界测试（空图/多起点/幽灵边/网关约束）
- expression.rs: 13 个新增求值测试（float/bool/string/复合表达式/空白容错）
- executor.rs: 3 个 is_join_gateway 纯函数测试

crates/erp-workflow/src/engine/executor.rs

@@ -627,3 +627,73 @@ impl FlowExecutor {
         Ok(())
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::dto::{EdgeDef, NodeDef, NodeType};
+
+    fn make_node(id: &str, node_type: NodeType) -> NodeDef {
+        NodeDef {
+            id: id.to_string(),
+            node_type,
+            name: id.to_string(),
+            assignee_id: None,
+            candidate_groups: None,
+            service_type: None,
+            service_config: None,
+            position: None,
+        }
+    }
+
+    fn make_edge(id: &str, source: &str, target: &str) -> EdgeDef {
+        EdgeDef {
+            id: id.to_string(),
+            source: source.to_string(),
+            target: target.to_string(),
+            condition: None,
+            label: None,
+        }
+    }
+
+    #[test]
+    fn test_is_join_gateway_with_multiple_incoming() {
+        let nodes = vec![
+            make_node("start", NodeType::StartEvent),
+            make_node("a", NodeType::UserTask),
+            make_node("b", NodeType::ServiceTask),
+            make_node("join", NodeType::ParallelGateway),
+            make_node("end", NodeType::EndEvent),
+        ];
+        let edges = vec![
+            make_edge("e1", "start", "a"),
+            make_edge("e2", "start", "b"),
+            make_edge("e3", "a", "join"),
+            make_edge("e4", "b", "join"),
+            make_edge("e5", "join", "end"),
+        ];
+        let graph = FlowGraph::build(&nodes, &edges);
+        assert!(FlowExecutor::is_join_gateway("join", &graph));
+    }
+
+    #[test]
+    fn test_is_not_join_gateway_single_incoming() {
+        let nodes = vec![
+            make_node("start", NodeType::StartEvent),
+            make_node("fork", NodeType::ParallelGateway),
+            make_node("end", NodeType::EndEvent),
+        ];
+        let edges = vec![
+            make_edge("e1", "start", "fork"),
+            make_edge("e2", "fork", "end"),
+        ];
+        let graph = FlowGraph::build(&nodes, &edges);
+        assert!(!FlowExecutor::is_join_gateway("fork", &graph));
+    }
+
+    #[test]
+    fn test_is_not_join_gateway_for_nonexistent_node() {
+        let graph = FlowGraph::build(&[], &[]);
+        assert!(!FlowExecutor::is_join_gateway("nonexistent", &graph));
+    }
+}