fix(genai): propagate the stop instance attribute to the model

[obchain (obchain)]

Why

Fixes #1586. stop is a documented model parameter on ChatGoogleGenerativeAI, but setting it at construction time is silently ignored:

llm = ChatGoogleGenerativeAI(model="gemini-2.5-flash-lite", stop=["model"])

# Wrong: stop sequence not applied -> full output
llm.invoke("Repeat after me: 'I am a model from Google.'")

# Works: stop passed per-call
llm.invoke("Repeat after me: 'I am a model from Google.'", stop=["model"])

Root cause

_build_base_generation_config sets stop_sequences directly from the per-call stop argument:

"stop_sequences": stop,

Every other parameter in that config falls back to its instance attribute (kwargs.get("temperature", self.temperature), self.max_output_tokens, self.top_k, ...). stop is the only one that does not, so self.stop is never consulted.

What

• Fall back to self.stop when no per-call stop is supplied:

  "stop_sequences": stop if stop is not None else self.stop,

• Add test_instance_stop_is_propagated confirming a model-level stop reaches the generation config. The existing test_kwargs_override_stop already covers the per-call-override path, which remains unchanged.

Areas requiring careful review

• Precedence is preserved: a per-call stop still overrides the instance attribute (the existing override test passes). The instance attribute now only applies when no per-call stop is given.

Disclaimer

AI agents were involved in drafting this contribution; the diagnosis, fix, and test were reviewed manually before submission.

[Sai Teja Bandaru (saitejabandaru-in)]

This is a solid and clean fix for the stop sequences propagation bug (#1586).

Using stop if stop is not None else self.stop correctly handles the case where stop is explicitly passed as an empty list [] at call time to override an instance-level self.stop configuration (which evaluates to [] and bypasses the fallback, as intended).

The unit test is comprehensive and covers the exact regression path. Verified the diff and this looks great!