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/langchain_ollama/chat_models.py

@@ -26,6 +26,7 @@ from langchain_core.messages import (
     AIMessageChunk,
     BaseMessage,
     BaseMessageChunk,
+    ChatMessage,
     HumanMessage,
     SystemMessage,
     ToolCall,
@@ -511,7 +512,7 @@ class ChatOllama(BaseChatModel):
     ) -> Sequence[Message]:
         ollama_messages: list = []
         for message in messages:
-            role: Literal["user", "assistant", "system", "tool"]
+            role: str
             tool_call_id: Optional[str] = None
             tool_calls: Optional[list[dict[str, Any]]] = None
             if isinstance(message, HumanMessage):
@@ -528,6 +529,8 @@ class ChatOllama(BaseChatModel):
                 )
             elif isinstance(message, SystemMessage):
                 role = "system"
+            elif isinstance(message, ChatMessage):
+                role = message.role
             elif isinstance(message, ToolMessage):
                 role = "tool"
                 tool_call_id = message.tool_call_id