test(backend): update employee service tests

- tests/test_employee_service.py: update employee service tests

server/tests/test_employee_service.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import pytest
 from sqlalchemy import create_engine, func, select
 from sqlalchemy.orm import Session, sessionmaker
 from sqlalchemy.pool import StaticPool
@@ -115,3 +116,12 @@ def test_disable_employee_marks_status_and_logs_change() -> None:
         assert persisted is not None
         assert persisted.employment_status == "停用"
         assert any(item.action == "停用员工账号" for item in updated.history)
+
+
+def test_update_employee_rejects_invalid_date_format() -> None:
+    with build_session() as db:
+        service = EmployeeService(db)
+        employee = service.list_employees()[0]
+
+        with pytest.raises(ValueError, match="出生日期格式必须为 YYYY-MM-DD。"):
+            service.update_employee(employee.id, EmployeeUpdate(birth_date="2024/01/01"))