fix(google-common): set function calling mode to auto when responseMimeType is application/json

[athan37]

Fixes 400 error when using structured output with Google models by setting function calling mode to "auto" instead of "any" when responseMimeType is "application/json".

"error" : {
   "code" 400,
   "message" : "Forced function calling (mode = ANY) is not supported when 'response_mime_type' is not set to 'text/plain'. Please set 'tool_config.function_calling_config.mode= AUTO' or set 'response_mime_type' to 'text/plain'. For more details, see: https://cloud.google.com/vertex-ai/generative-ai/docs/multimodal/function-calling#tool-config",
   status: "INVALID_ARGUMENT"
}

Changes

• Modified formatToolConfig in gemini.ts to dynamically set function calling mode based on responseMimeType
• Enhanced withStructuredOutput in chat_models.ts to automatically set responseMimeType: "application/json"
• Added unit tests covering both default and override scenarios
• Updated documentation to reflect JSON mode support for ChatVertexAI and ChatGoogleGenerativeAI

Testing

• Added 2 unit tests

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[christian-bromann]

Really appreciate you taking the time to open this PR, @athan37 🙏
The team is currently focused on shipping our v1 release, so we’re pausing detailed reviews for a bit. We’ll follow up within the next 2–4 weeks once things settle — thanks so much for your patience!