googleapi/vertex: fix multi-tool call error, Invalid argument

[CloudAceEmma]

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This PR fix Invalid argument error when using multiple tools, google genai only support one tool, multiple tools must be embedded into function declarations, cc @eliben, see also:
GoogleCloudPlatform/generative-ai#636
https://cloud.google.com/vertex-ai/generative-ai/docs/multimodal/function-calling#chat-samples

[eliben]

Thank you for the contribution!

initial comments

[eliben]

LGTM

@tmc to merge