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
#30147 (comment).

Furthermore, ollama-python is removing the limitations on valid roles
that can be passed through chat messages to a model in ollama -
ollama/ollama-python#462 (comment).

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](
ollama/ollama-python#462 (comment)), 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
ollama/ollama#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 <[email protected]>
Co-authored-by: Chester Curme <[email protected]>

Author: 3 people