fix(ollama): raise error when clients are not initialized (#35185)

## Summary
- When `self._client` is `None` in `_create_chat_stream()`, the method
silently produces an empty generator instead of failing.
- The error only surfaces later as a misleading `"No data received from
Ollama stream"` ValueError, making it difficult to diagnose the actual
root cause (uninitialized client).
- Changed to raise `RuntimeError` immediately with a clear message when
the sync client is not initialized.

## Why this matters
Users who hit this path see a confusing error message that points them
in the wrong direction. An explicit error at the point of failure makes
debugging straightforward.

## Test plan
- [x] Added `test_create_chat_stream_raises_when_client_none`
- [x] Existing tests still pass

> This PR was authored with the help of AI tools.

---------

Co-authored-by: Mason Daugherty <github@mdrxy.com>

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

@@ -449,6 +449,42 @@ 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  # type: ignore[assignment]
+
+        with pytest.raises(RuntimeError, match="sync client is not initialized"):
+            list(llm._create_chat_stream([HumanMessage("Hello")]))
+
+
+async def test_acreate_chat_stream_raises_when_client_none() -> None:
+    """Test that _acreate_chat_stream raises RuntimeError when client is None."""
+    with patch("langchain_ollama.chat_models.AsyncClient") as mock_client_class:
+        mock_client_class.return_value = MagicMock()
+        llm = ChatOllama(model="test-model")
+        # Force _async_client to None to simulate uninitialized state
+        llm._async_client = None  # type: ignore[assignment]
+
+        with pytest.raises(RuntimeError, match="async client is not initialized"):
+            async for _ in llm._acreate_chat_stream([HumanMessage("Hello")]):
+                pass
+
+
+def test_invoke_raises_when_client_none() -> None:
+    """Test that RuntimeError propagates through the public invoke() API."""
+    with patch("langchain_ollama.chat_models.Client") as mock_client_class:
+        mock_client_class.return_value = MagicMock()
+        llm = ChatOllama(model="test-model")
+        llm._client = None  # type: ignore[assignment]
+
+        with pytest.raises(RuntimeError, match="sync client is not initialized"):
+            llm.invoke([HumanMessage("Hello")])
+
+
 def test_chat_ollama_ignores_strict_arg() -> None:
     """Test that ChatOllama ignores the 'strict' argument."""
     response = [

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

@@ -1,7 +1,9 @@
 """Test embedding model integration."""
 
 from typing import Any
-from unittest.mock import Mock, patch
+from unittest.mock import MagicMock, Mock, patch
+
+import pytest
 
 from langchain_ollama.embeddings import OllamaEmbeddings
 
@@ -50,3 +52,25 @@ def test_embed_documents_passes_options(mock_client_class: Any) -> None:
     options = call_args.kwargs["options"]
     assert options["num_gpu"] == 4
     assert options["temperature"] == 0.5
+
+
+def test_embed_documents_raises_when_client_none() -> None:
+    """Test that embed_documents raises RuntimeError when client is None."""
+    with patch("langchain_ollama.embeddings.Client") as mock_client_class:
+        mock_client_class.return_value = MagicMock()
+        embeddings = OllamaEmbeddings(model="test-model")
+        embeddings._client = None  # type: ignore[assignment]
+
+        with pytest.raises(RuntimeError, match="sync client is not initialized"):
+            embeddings.embed_documents(["test"])
+
+
+async def test_aembed_documents_raises_when_client_none() -> None:
+    """Test that aembed_documents raises RuntimeError when async client is None."""
+    with patch("langchain_ollama.embeddings.AsyncClient") as mock_client_class:
+        mock_client_class.return_value = MagicMock()
+        embeddings = OllamaEmbeddings(model="test-model")
+        embeddings._async_client = None  # type: ignore[assignment]
+
+        with pytest.raises(RuntimeError, match="async client is not initialized"):
+            await embeddings.aembed_documents(["test"])

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

@@ -1,7 +1,9 @@
 """Test Ollama Chat API wrapper."""
 
 from typing import Any
-from unittest.mock import patch
+from unittest.mock import MagicMock, patch
+
+import pytest
 
 from langchain_ollama import OllamaLLM
 
@@ -65,3 +67,26 @@ def test_reasoning_aggregation() -> None:
         result.generations[0][0].generation_info["thinking"]
         == "I am thinking. Still thinking."
     )
+
+
+def test_create_generate_stream_raises_when_client_none() -> None:
+    """Test that _create_generate_stream raises RuntimeError when client is None."""
+    with patch("langchain_ollama.llms.Client") as mock_client_class:
+        mock_client_class.return_value = MagicMock()
+        llm = OllamaLLM(model="test-model")
+        llm._client = None  # type: ignore[assignment]
+
+        with pytest.raises(RuntimeError, match="sync client is not initialized"):
+            list(llm._create_generate_stream("Hello"))
+
+
+async def test_acreate_generate_stream_raises_when_client_none() -> None:
+    """Test that _acreate_generate_stream raises RuntimeError when client is None."""
+    with patch("langchain_ollama.llms.AsyncClient") as mock_client_class:
+        mock_client_class.return_value = MagicMock()
+        llm = OllamaLLM(model="test-model")
+        llm._async_client = None  # type: ignore[assignment]
+
+        with pytest.raises(RuntimeError, match="async client is not initialized"):
+            async for _ in llm._acreate_generate_stream("Hello"):
+                pass