fix(google-common): pass structured objects directly in FunctionResponse

[Jackjin (jackjin1997)]

Summary

• Fix toolMessageToContent in @langchain/google-common to pass structured objects directly to the Gemini API's FunctionResponse.response field instead of double-serializing them
• The Gemini API accepts any JSON object as the response (Struct format), but the previous code always wrapped parsed content in { content: parsed }, which added an unnecessary nesting layer
• Now correctly handles: JSON string content (parsed and passed directly), plain string content (wrapped in { result } ), JSON arrays, and ContentBlock arrays

Why

The Gemini API docs specify that FunctionResponse.response accepts any arbitrary JSON object directly. The previous implementation unnecessarily stringified and re-parsed content, and wrapped results in { content: ... } which doesn't match the API's expected format. This caused issues when tools return structured JSON — the model received stringified JSON instead of the actual object structure.

Test plan

• Added unit tests for JSON string content → parsed object in response
• Added unit tests for plain string content → { result: string }
• Added unit tests for JSON array content → { result: array }
• All 43 existing + new tests pass

Fixes #10439

This PR was developed with assistance from an AI coding agent.

[changeset-bot]

🦋 Changeset detected

Latest commit: 5fa232b

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 6 packages

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

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR