fix(rag): pass gcs_path to quickstart entrypoint

[aniruddhaadak80]

Description

Fixes #14610

The RAG quickstart entry point passed a paths keyword to quickstart, whose signature accepts gcs_path. The sample therefore raised TypeError before making an API request. This change passes the GCS path using the supported argument and removes the unused Drive placeholder from that entry point.

A dependency-free regression test executes the entry-point call with the real function signature, so the pre-fix keyword mismatch fails before the API is reached.

Validation:

• python -m pytest generative_ai/rag/test_quickstart_entrypoint.py -q — 1 passed
• Pre-fix entry-point execution — reproduced TypeError: quickstart() got an unexpected keyword argument 'paths'
• python -m ruff check generative_ai/rag/quickstart_example.py generative_ai/rag/test_quickstart_entrypoint.py — passed
• python -m mypy --follow-imports=skip --ignore-missing-imports generative_ai/rag/quickstart_example.py — no issues found
• python -m black --check generative_ai/rag/test_quickstart_entrypoint.py — passed
• Live GCP execution was not run because this fix is exercised without credentials.

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 updates the entry point in quickstart_example.py to pass gcs_path instead of a list of paths containing a Google Drive path. Additionally, a new test file test_quickstart_entrypoint.py is introduced to verify that the entry point correctly invokes the quickstart function with the expected arguments using AST parsing and execution. There are no review comments, and I have no feedback to provide.