community[patch]: fix structured_output in llamacpp integration (#27202)

Resolves https://github.com/langchain-ai/langchain/issues/25318.

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

@@ -0,0 +1,19 @@
+from pydantic import BaseModel, Field
+
+from langchain_community.chat_models import ChatLlamaCpp
+
+
+class Joke(BaseModel):
+    """Joke to tell user."""
+
+    setup: str = Field(description="question to set up a joke")
+    punchline: str = Field(description="answer to resolve the joke")
+
+
+# TODO: replace with standard integration tests
+# See example in tests/integration_tests/chat_models/test_litellm.py
+def test_structured_output() -> None:
+    llm = ChatLlamaCpp(model_path="/path/to/Meta-Llama-3.1-8B-Instruct.Q4_K_M.gguf")
+    structured_llm = llm.with_structured_output(Joke)
+    result = structured_llm.invoke("Tell me a short joke about cats.")
+    assert isinstance(result, Joke)