fix(google-genai): merge consecutive function-role messages in convertBaseMessagesToContent

[Mohammad Jafari (mossein)]

Fixes #10342

When multiple tool calls are executed in parallel, convertBaseMessagesToContent receives consecutive ToolMessages that all map to the function role. The existing code throws "Google Generative AI requires alternate messages between authors" instead of merging them.

This patch:

1. Fixes an off-by-one bug (acc.content[acc.content.length] → acc.content[acc.content.length - 1]) that prevented the duplicate-role check from ever triggering.
2. When consecutive function-role messages are detected, merges their functionResponse parts into the previous content entry instead of throwing. Gemini expects all functionResponse parts for a given model turn to live in a single user turn.

[Mohammad Jafari (mossein)]

cc Hunter Lovell (@hntrl) Jacob Lee (@jacoblee93), this fixes a bug that causes Gemini to hang when LangChain sends parallel tool results as separate user turns instead of merging them into one. Happy to adjust anything if needed.

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Latest commit: de97364

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Name	Type
@langchain/google-genai	Patch
@langchain/google-common	Patch
@langchain/google-gauth	Patch
@langchain/google-webauth	Patch
@langchain/google-vertexai	Patch
@langchain/google-vertexai-web	Patch

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[pawel-twardziak]

hi Mohammad Jafari (@mossein)

great work! I think it would be good to use actualRole in the merge condition, e.g. compute actualRole before the prevContent check and use prevContent.role === actualRole and then role === "function" to merge

and add unit tests

[Mohammad Jafari (mossein)]

hi Mohammad Jafari (Mohammad Jafari (@mossein))

great work! I think it would be good to use actualRole in the merge condition, e.g. compute actualRole before the prevContent check and use prevContent.role === actualRole and then role === "function" to merge

and add unit tests

Good call pawel-twardziak. I moved the actualRole computation before the prevContent check so the comparison uses the mapped API role.
Also added unit tests covering parallel tool calls with 2 and 3 consecutive ToolMessages. Let me know if anything else needs adjusting.

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[pawel-twardziak]

LGTM, thanks Mohammad Jafari (@mossein) 💪

[Mohammad Jafari (mossein)]

Hey Christian Bromann (@christian-bromann) - this fixes the same class of bug as #10343 (which was merged) but for the @langchain/google-genai package. Would love a review when you get a chance!

[Hunter Lovell (hntrl)]

thanks for the fix!