feat: 异常树 error_code 类属性（对齐 api-design §7 错误码）

tests/test_inference_errors.py

@@ -18,4 +18,18 @@ def test_error_hierarchy():
 
 def test_error_message_roundtrip():
     e = LLMTimeoutError("timeout!")
-    assert str(e) == "timeout!"
+    assert str(e) == "timeout!"
+
+
+def test_error_code_class_attributes():
+    # error_code 与 api-design §7 错误码表一一对应（机器可读事实源）
+    assert LLMError.error_code is None
+    assert LLMTimeoutError.error_code == "LLM_TIMEOUT"
+    assert LLMNetworkError.error_code == "LLM_NETWORK_ERROR"
+    assert LLMNotConfiguredError.error_code == "LLM_NOT_CONFIGURED"
+    assert LLMResponseError.error_code == "LLM_PARSE_ERROR"
+
+
+def test_error_code_inherited_by_instance():
+    e = LLMNetworkError("network")
+    assert e.error_code == "LLM_NETWORK_ERROR"