ollama: add validate_model_on_init, catch more errors (#31784)

* Ensure access to local model during `ChatOllama` instantiation
(#27720). This adds a new param `validate_model_on_init` (default:
`true`)
* Catch a few more errors from the Ollama client to assist users

libs/partners/ollama/tests/integration_tests/chat_models/test_chat_models_standard.py

@@ -1,8 +1,13 @@
 """Test chat model integration using standard integration tests."""
 
+from unittest.mock import MagicMock, patch
+
 import pytest
+from httpx import ConnectError
 from langchain_core.language_models import BaseChatModel
 from langchain_tests.integration_tests import ChatModelIntegrationTests
+from ollama import ResponseError
+from pydantic import ValidationError
 
 from langchain_ollama.chat_models import ChatOllama
 
@@ -47,3 +52,29 @@ class TestChatOllama(ChatModelIntegrationTests):
     )
     async def test_tool_calling_async(self, model: BaseChatModel) -> None:
         await super().test_tool_calling_async(model)
+
+    @patch("langchain_ollama.chat_models.Client.list")
+    def test_init_model_not_found(self, mock_list: MagicMock) -> None:
+        """Test that a ValueError is raised when the model is not found."""
+        mock_list.side_effect = ValueError("Test model not found")
+        with pytest.raises(ValueError) as excinfo:
+            ChatOllama(model="non-existent-model", validate_model_on_init=True)
+        assert "Test model not found" in str(excinfo.value)
+
+    @patch("langchain_ollama.chat_models.Client.list")
+    def test_init_connection_error(self, mock_list: MagicMock) -> None:
+        """Test that a ValidationError is raised on connect failure during init."""
+        mock_list.side_effect = ConnectError("Test connection error")
+
+        with pytest.raises(ValidationError) as excinfo:
+            ChatOllama(model="any-model", validate_model_on_init=True)
+        assert "not found in Ollama" in str(excinfo.value)
+
+    @patch("langchain_ollama.chat_models.Client.list")
+    def test_init_response_error(self, mock_list: MagicMock) -> None:
+        """Test that a ResponseError is raised."""
+        mock_list.side_effect = ResponseError("Test response error")
+
+        with pytest.raises(ValidationError) as excinfo:
+            ChatOllama(model="any-model", validate_model_on_init=True)
+        assert "Received an error from the Ollama API" in str(excinfo.value)