feat(vertex-ai): add sample for evaluation/example-syntax

[Valeriy-Burlaka]

Description

Convert and fix the Markdown sample from the "Evaluation API -> Example Syntax" section.
The original "sample" is not runnable in its current state, as it contains syntax errors and also omits important request details.

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/evaluation/example_syntax.py:24, tag generativeaionvertexai_evaluation_example_syntax

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[glasnt]

Due to the age of this PR, it needs to be rebased to get Python 3.13 tests to start.