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/error.rs

@@ -51,3 +51,78 @@ impl From<WorkflowError> for AppError {
 }
 
 pub type WorkflowResult<T> = Result<T, WorkflowError>;
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn validation_maps_to_app_error_validation() {
+        let err = WorkflowError::Validation("字段缺失".to_string());
+        let app: AppError = err.into();
+        match app {
+            AppError::Validation(msg) => assert!(msg.contains("字段缺失")),
+            other => panic!("期望 AppError::Validation，得到 {:?}", other),
+        }
+    }
+
+    #[test]
+    fn not_found_maps_to_app_error_not_found() {
+        let err = WorkflowError::NotFound("流程不存在".to_string());
+        let app: AppError = err.into();
+        match app {
+            AppError::NotFound(msg) => assert!(msg.contains("流程不存在")),
+            other => panic!("期望 AppError::NotFound，得到 {:?}", other),
+        }
+    }
+
+    #[test]
+    fn duplicate_definition_maps_to_app_error_conflict() {
+        let err = WorkflowError::DuplicateDefinition("key 已存在".to_string());
+        let app: AppError = err.into();
+        match app {
+            AppError::Conflict(msg) => assert!(msg.contains("key 已存在")),
+            other => panic!("期望 AppError::Conflict，得到 {:?}", other),
+        }
+    }
+
+    #[test]
+    fn invalid_diagram_maps_to_validation() {
+        let err = WorkflowError::InvalidDiagram("缺少 StartEvent".to_string());
+        let app: AppError = err.into();
+        match app {
+            AppError::Validation(msg) => assert!(msg.contains("缺少 StartEvent")),
+            other => panic!("期望 AppError::Validation，得到 {:?}", other),
+        }
+    }
+
+    #[test]
+    fn invalid_state_maps_to_validation() {
+        let err = WorkflowError::InvalidState("流程已结束".to_string());
+        let app: AppError = err.into();
+        match app {
+            AppError::Validation(msg) => assert!(msg.contains("流程已结束")),
+            other => panic!("期望 AppError::Validation，得到 {:?}", other),
+        }
+    }
+
+    #[test]
+    fn expression_error_maps_to_validation() {
+        let err = WorkflowError::ExpressionError("语法错误".to_string());
+        let app: AppError = err.into();
+        match app {
+            AppError::Validation(msg) => assert!(msg.contains("语法错误")),
+            other => panic!("期望 AppError::Validation，得到 {:?}", other),
+        }
+    }
+
+    #[test]
+    fn version_mismatch_maps_directly() {
+        let err = WorkflowError::VersionMismatch;
+        let app: AppError = err.into();
+        match app {
+            AppError::VersionMismatch => {},
+            other => panic!("期望 AppError::VersionMismatch，得到 {:?}", other),
+        }
+    }
+}