Use new enterprise env variable for Gemini calls

[jscud]

Description

Fixes #14198

Note: Before submitting a pull request, please open an issue for discussion if you are not associated with Google.

Checklist

• I have followed Sample Guidelines from AUTHORING_GUIDE.MD
• README is updated to include all relevant information
• Tests pass: nox -s py-3.9 (see Test Environment Setup)
• Lint pass: nox -s lint (see Test Environment Setup)
• These samples need a new API enabled in testing projects to pass (let us know which ones)
• These samples need a new/updated env vars in testing projects set to pass (let us know which ones)

Changes GOOGLE_GENAI_USE_VERTEXAI to GOOGLE_GENAI_USE_ENTERPRISE

• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• Please merge this PR for me once it is approved

[gemini-code-assist]

Code Review

This pull request updates the environment variable key from GOOGLE_GENAI_USE_VERTEXAI to GOOGLE_GENAI_USE_ENTERPRISE in the test file. Feedback indicates that setting environment variables at the module level can cause side effects by modifying global state and suggests that the specific variable is unused in this context, recommending its removal or the use of a scoped fixture like monkeypatch for better test isolation.

[gemini-code-assist]

Setting environment variables at the module level in tests modifies global state and can cause side effects. Additionally, 'GOOGLE_GENAI_USE_ENTERPRISE' is not used by 'templatefolder_with_txt.py', making this dead code in this context. For a template, it is better to remove unnecessary boilerplate or use a scoped fixture (e.g., 'monkeypatch') to ensure test isolation.