refactor(genai/labels): replace vertexai.generative_models with genai

[XrossFox]

Description

Update the codebase to use the new genai SDK replacement for all legacy vertexai.generative_models imports and usage.

Fixes b/537370586

Checklist

Testing

• I have tested this change on a live environment and verified it works as intended.

Compliance & Style

• I have followed the Sample Guidelines.
• The README is updated with all relevant information (if applicable).
• If this PR adds a new sample directory: I updated the CODEOWNERS file.

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Post-Approval Actions

• Please merge this PR for me once it is approved

[gemini-code-assist]

Code Review

This pull request introduces a new Python example demonstrating how to set labels when generating content with the google-genai SDK, along with associated tests and configuration files. Feedback is provided to correct a non-existent version of google-genai in requirements.txt, expand test coverage by not ignoring stable Python versions (3.11, 3.12, 3.13) in noxfile_config.py, and strengthen the test assertion to verify that response.text is returned.

[XrossFox]

This sample has been tested locally:

[angelcaamal]

LGTM!

[snippet-bot]

Here is the summary of changes.

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• genai/labels/labels_example.py:15, tag aiplatform_genai_gemini_set_labels

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