feat: 结果对象 error_code 字段（默认 None 向后兼容）

tests/test_inference_types.py

@@ -35,6 +35,22 @@ def test_structured_result_status_parse_error():
     assert r.status == "parse_error" and r.error == "bad json"
 
 
+def test_chat_result_error_code_default_none():
+    r = ChatResult(
+        text="t", model="m", prompt_version="v1", usage=TokenUsage(),
+        duration_ms=10, status="ok",
+    )
+    assert r.error_code is None
+
+
+def test_structured_result_error_code_default_none():
+    r = StructuredResult(
+        data={"a": 1}, raw_text='{"a":1}', parse_attempts=1, model="m",
+        prompt_version="v1", usage=TokenUsage(), duration_ms=10, status="ok",
+    )
+    assert r.error_code is None
+
+
 def test_prompt_fields():
     p = Prompt(name="writer", version="v2", template="章节 {{chapter}}")
     assert p.name == "writer" and p.version == "v2"