fix: lint 修复 + api_error 测试更新

- mail_notify: f-string 反斜杠修复、行过长修复、unused import
- test_classify_outcome: api_error should_retry 改 True

tests/unit/test_classify_outcome.py

@@ -165,14 +165,16 @@ class TestClassifyErrorApi:
             1, {"status": "error"}, "rate_limit exceeded", None
         )
         assert result["outcome"] == "api_error"
-        assert result["should_retry"] is False
+        assert result["should_retry"] is True
+        assert result["cooldown_seconds"] == 60
 
     def test_stderr_500(self):
         result = Spawner._classify_outcome(
             1, {"status": "error"}, "HTTP 500 Internal Server Error", None
         )
         assert result["outcome"] == "api_error"
-        assert result["should_retry"] is False
+        assert result["should_retry"] is True
+        assert result["cooldown_seconds"] == 60
 
 
 class TestClassifyErrorCompact: