community[patch]: Improve ChatBaichuan init args and role (#23878)

- **Description:** Improve ChatBaichuan init args and role
   -  ChatBaichuan adds `system` role
   - alias: `baichuan_api_base` -> `base_url`
   - `with_search_enhance` is deprecated
   - Add `max_tokens` argument

libs/community/tests/integration_tests/chat_models/test_baichuan.py

@@ -76,3 +76,14 @@ async def test_chat_baichuan_astream() -> None:
     async for chunk in chat.astream("今天天气如何？"):
         assert isinstance(chunk, AIMessage)
         assert isinstance(chunk.content, str)
+
+
+def test_chat_baichuan_with_system_role() -> None:
+    chat = ChatBaichuan()  # type: ignore[call-arg]
+    messages = [
+        ("system", "你是一名专业的翻译家，可以将用户的中文翻译为英文。"),
+        ("human", "我喜欢编程。"),
+    ]
+    response = chat.invoke(messages)
+    assert isinstance(response, AIMessage)
+    assert isinstance(response.content, str)