langchain-ollama (partners) / langchain-core: allow passing ChatMessages to Ollama (including arbitrary roles) (#30411)

Replacement for PR #30191 (@ccurme)

**Description**: currently, ChatOllama [will raise a value error if a
ChatMessage is passed to
it](https://github.com/langchain-ai/langchain/blob/master/libs/partners/ollama/langchain_ollama/chat_models.py#L514),
as described
https://github.com/langchain-ai/langchain/pull/30147#issuecomment-2708932481.

Furthermore, ollama-python is removing the limitations on valid roles
that can be passed through chat messages to a model in ollama -
https://github.com/ollama/ollama-python/pull/462#event-16917810634.

This PR removes the role limitations imposed by langchain and enables
passing langchain ChatMessages with arbitrary 'role' values through the
langchain ChatOllama class to the underlying ollama-python Client.

As this PR relies on [merged but unreleased functionality in
ollama-python](
https://github.com/ollama/ollama-python/pull/462#event-16917810634), I
have temporarily pointed the ollama package source to the main branch of
the ollama-python github repo.

Format, lint, and tests of new functionality passing. Need to resolve
issue with recently added ChatOllama tests. (Now resolved)

**Issue**: resolves #30122 (related to ollama issue
https://github.com/ollama/ollama/issues/8955)

**Dependencies**: no new dependencies

[x] PR title
[x] PR message
[x] Lint and test: format, lint, and test all running successfully and
passing

---------

Co-authored-by: Ryan Stewart <ryanstewart@Ryans-MacBook-Pro.local>
Co-authored-by: Chester Curme <chester.curme@gmail.com>

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

@@ -1,7 +1,13 @@
 """Test chat model integration."""
 
 import json
+from collections.abc import Generator
+from contextlib import contextmanager
+from typing import Any
 
+import pytest
+from httpx import Client, Request, Response
+from langchain_core.messages import ChatMessage
 from langchain_tests.unit_tests import ChatModelUnitTests
 
 from langchain_ollama.chat_models import ChatOllama, _parse_arguments_from_tool_call
@@ -23,3 +29,38 @@ def test__parse_arguments_from_tool_call() -> None:
     response = _parse_arguments_from_tool_call(raw_tool_calls[0])
     assert response is not None
     assert isinstance(response["arg_1"], str)
+
+
+@contextmanager
+def _mock_httpx_client_stream(
+    *args: Any, **kwargs: Any
+) -> Generator[Response, Any, Any]:
+    yield Response(
+        status_code=200,
+        content='{"message": {"role": "assistant", "content": "The meaning ..."}}',
+        request=Request(method="POST", url="http://whocares:11434"),
+    )
+
+
+def test_arbitrary_roles_accepted_in_chatmessages(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    monkeypatch.setattr(Client, "stream", _mock_httpx_client_stream)
+
+    llm = ChatOllama(
+        base_url="http://whocares:11434",
+        model="granite3.2",
+        verbose=True,
+        format=None,
+    )
+
+    messages = [
+        ChatMessage(
+            role="somerandomrole",
+            content="I'm ok with you adding any role message now!",
+        ),
+        ChatMessage(role="control", content="thinking"),
+        ChatMessage(role="user", content="What is the meaning of life?"),
+    ]
+
+    llm.invoke(messages)