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

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

[snippet-bot[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

---

This comment is generated by snippet-bot. If you find problems with this result, please file an issue at: https://github.com/googleapis/repo-automation-bots/issues. To update this comment, add snippet-bot:force-run label or use the checkbox below:

• [ ] Refresh this comment

[glasnt]

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

[msampathkumar]

Putting a hold on all Generative AI samples development.

Thank you for your work.