feat(genai/model_tuning) Migrate samples to genai/model_tuning using vertexai.language_models.

[XrossFox]

Refactored tests to wrok with new changes.

Description

This PR migrates model tuning examples to use the most up-to-date features from the google-genai and google-cloud-aiplatform
libraries.

Changes:

• feat: Added pretrained_codegen_example.py demonstrating supervised fine-tuning using google-genai.
• feat: Added evaluate_model_example.py demonstrating model evaluation using vertexai.evaluation.
• test: Added corresponding test files (pretrained_examples_test.py and evaluate_model_example_test.py) with cleanup logic.
• chore: Created noxfile_config.py, requirements.txt, and requirements-test.txt for the new genai/model_tuning
  directory.

Fixes b/540948584

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

[XrossFox]

Tested locally:

[gemini-code-assist]

Code Review

This pull request introduces code examples and tests for evaluating and tuning Gemini models using the Vertex AI and Google GenAI SDKs. The feedback recommends updating the Nox configuration to include Python 3.13 in tests, increasing the backoff retry duration to handle rate limits during model evaluation, and wrapping the tuning test's cleanup logic in a try...finally block to prevent resource leaks on assertion failures.