feat(vertex-ai): function-calling api: convert 'example syntax' from markdown

[Valeriy-Burlaka]

Description

Convert the Markdown sample from the "Function Calling API -> Example Syntax" section.
The original "sample" is not runnable in its current state, as it doesn't use real values and omits details like imports, function body, etc..

Checklist

• I have followed Sample Guidelines from AUTHORING_GUIDE.MD
• Tests pass: nox -s py-3.9 && nox -s py-3.12
• Lint pass: nox -s lint

[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• generative_ai/function_calling/example_syntax.py:26, tag generativeaionvertexai_function_calling_example_syntax

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[Valeriy-Burlaka]

Merged with #12811