refactor(ch1-4): align filenames and update corresponding README sections

m04_langchain_basics/s03_chain.py

@@ -0,0 +1,27 @@
+from config import OPENAI_API_KEY
+from langchain_core.prompts import ChatPromptTemplate
+from langchain_openai import ChatOpenAI
+from langchain_core.output_parsers import StrOutputParser
+
+# 定义提示词模板
+prompt = ChatPromptTemplate.from_messages([
+    ("system", "你非常可爱，说话末尾会带个喵"),
+    ("human", "{input}")  # {input}:占位符
+])
+
+# 初始化模型
+llm = ChatOpenAI(
+    model="deepseek-chat",
+    api_key=OPENAI_API_KEY,
+    base_url="https://api.deepseek.com"
+)
+
+# 定义解析器，把LLM返回的AIMessage转成字符串
+parser = StrOutputParser()
+
+# 组成Chain
+chain = prompt | llm | parser
+
+# 最终调用
+result = chain.invoke({"input": "你好喵"})
+print(result)