Add Model Name and System Fingerprint to llm_output in _convert_response_to_chat_result

[jmoreno11]

This PR enhances _convert_response_to_chat_result by including the model name and system fingerprint in llm_output. The model name is particularly important for observability tools like LangFuse, which rely on llm_output["model_name"] for tracking.

Currently, Databricks does not provide the model name in the metadata—only the endpoint name. This change ensures that external models are properly identified, improving observability and logging.

Changes:

• Added model_name to llm_output, defaulting to self.model if not in the response.
• Added system_fingerprint to llm_output for additional metadata.
• Updated llm_output to include token_usage separately for better structure.

Why?
When using external models with Databricks, the returned metadata only includes the endpoint name, not the actual model name. This change allows users to extract the correct model name for logging and monitoring.

Testing:

• Ensured the new metadata fields are correctly populated when present in the response.
• Maintains backward compatibility by defaulting to self.model if response["model"] is missing.

[sunishsheth2009]

Can we rename this to usage so it doesn't change the API for existing users?

Suggested change

	"token_usage": response.get("usage", {}),
	"usage": response.get("usage", {}),

[jmoreno11]

I did as @bbqiu suggested, so usage remains intact

[jmoreno11]

I also created a new unit test, since the existing ones were not altered by this change

[sunishsheth2009]

Can we also update tests here: https://github.com/databricks/databricks-ai-bridge/blob/main/integrations/langchain/tests/unit_tests/test_chat_models.py#L84

Thank you for contributing. :)

[bbqiu]

lgtm!

[jmoreno11]

Please reapprove, I fixed the linting error (ruff --fix)

P.S. I'm confused about this particular linting error: did you change the linting configuration recently? Otherwise it doesn't make sense to me

[jmoreno11]

Apologies, ruff cli was doing something weird. I created a PR on my forked branch and the linting is now fixed.

[bbqiu]

approving to unblock merge