feat(retail): add Vertex AI Search for commerce snippets

[iennae]

• Introduce search_request, search_pagination, and search_offset samples (use cases as per https://docs.cloud.google.com/retail/docs/search-basic) resolves the python portion of b/268120562 (the java samples are still embedded)
• Use the correct "Vertex AI Search for commerce" product naming.
• Hardcode catalog and branch IDs to "default_catalog" and "default_branch" to maximize sample readability.
• Include unit tests with mocks and a shared conftest.py.
• Add a README.md detailing API prerequisites, ADC setup, and required IAM roles.
• Ensure all samples and tests are PEP8 compliant and formatted with Black.

Description

Replaces #13119

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 retail.googleapis.com; probably need a fixture for integration
• These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• Please merge this PR for me once it is approved

[snippet-bot]

Here is the summary of changes.

You are about to add 3 region tags.

• retail/snippets/search_offset.py:15, tag retail_v2_search_offset
• retail/snippets/search_pagination.py:15, tag retail_v2_search_pagination
• retail/snippets/search_request.py:15, tag retail_v2_search_request

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[gemini-code-assist]

Code Review

This pull request introduces Python code snippets and unit tests for Vertex AI Search for commerce, specifically demonstrating basic search requests, pagination, and result offsets. The review feedback highlights a critical logic issue in the pagination and offset implementations: iterating directly over the response pager triggers the client library's auto-pagination feature, which bypasses the manual pagination logic intended for the samples. To resolve this, the reviewer suggests using the .pages property to handle results page-by-page and provides necessary updates for the associated unit test mocks.