fix(ollama): raise error when sync client is not initialized

When self._client is None, _create_chat_stream silently produces an
empty generator. The error only surfaces later as a misleading
"No data received from Ollama stream" message, making it difficult
to diagnose the root cause.

Now raises RuntimeError immediately with a clear message when the sync
client is not initialized.

libs/partners/ollama/langchain_ollama/chat_models.py

@@ -948,12 +948,17 @@ class ChatOllama(BaseChatModel):
         stop: list[str] | None = None,
         **kwargs: Any,
     ) -> Iterator[Mapping[str, Any] | str]:
+        if not self._client:
+            msg = (
+                "Ollama sync client is not initialized. "
+                "Make sure the model was properly constructed."
+            )
+            raise RuntimeError(msg)
         chat_params = self._chat_params(messages, stop, **kwargs)
 
         if chat_params["stream"]:
-            if self._client:
-                yield from self._client.chat(**chat_params)
-        elif self._client:
+            yield from self._client.chat(**chat_params)
+        else:
             yield self._client.chat(**chat_params)
 
     def _chat_stream_with_aggregation(

libs/partners/ollama/tests/unit_tests/test_chat_models.py

@@ -449,6 +449,18 @@ def test_reasoning_param_passed_to_client() -> None:
         assert call_kwargs["think"] is True
 
 
+def test_create_chat_stream_raises_when_client_none() -> None:
+    """Test that _create_chat_stream raises RuntimeError when client is None."""
+    with patch("langchain_ollama.chat_models.Client") as mock_client_class:
+        mock_client_class.return_value = MagicMock()
+        llm = ChatOllama(model="test-model")
+        # Force _client to None to simulate uninitialized state
+        llm._client = None
+
+        with pytest.raises(RuntimeError, match="sync client is not initialized"):
+            list(llm._create_chat_stream([HumanMessage("Hello")]))
+
+
 def test_chat_ollama_ignores_strict_arg() -> None:
     """Test that ChatOllama ignores the 'strict' argument."""
     response = [