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

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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: https://github.com/GoogleCloudPlatform/generative-ai/issues/636 https://cloud.google.com/vertex-ai/generative-ai/docs/multimodal/function-calling#chat-samples

tmc / langchaingo