langchain-ollama (partners): allow passing ChatMessages to Ollama (including arbitrary roles)

[rylativity]

Replacement for PR #30191 (@ccurme)

Description: currently, ChatOllama will raise a value error if a ChatMessage is passed to it, 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, 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

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Name	Status	Preview	Comments	Updated (UTC)
langchain	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Mar 21, 2025 2:39pm

[rylativity]

@ccurme The change to libs/standard-tests/langchain_tests/unit_tests/chat_models.py in #30385 on 3/20 causes ChatOllama to fail two of the shared chatmodel tests (example test failure - https://github.com/langchain-ai/langchain/actions/runs/13985107639/job/39157371264). Even on the master branch, these tests fail in for ChatOllama (can you confirm?).

I've updated ChatOllama's with_structured_output method to accept the strict param, but I don't believe ollama actually does anything with that param currently (so I noted this in the docstring).

I have added the tests and example for using ChatMessages (optionally with arbitrary roles) with ChatOllama in the API reference.

As mentioned in the PR opening comment, this relies on functionality in ollama-python that is merged into main but not yet in a versioned release. We will need to revert the change to libs/partners/ollama/pyproject.toml before merging, but for now I've left it in for the CI/CD checks.