feat(oa): reject invalid dept form_data on start

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/app/services/oa_engine.py

@@ -1550,6 +1550,12 @@ def start_instance(
     except FormulaError as e:
         raise OaEngineError(f"公式计算失败: {e}") from e
 
+    from app.services.oa_dept_select import validate_dept_form_values
+
+    dept_errs = validate_dept_form_values(schema, data)
+    if dept_errs:
+        raise OaEngineError(dept_errs[0])
+
     user = (
         db.query(SysUser)
         .filter(SysUser.id == initiator_user_id, SysUser.del_flag == 0)