test(bigframes): Use gemini-flash-2.5 in place of deprecated 2.0

[TrevorBergeron]

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Fixes #<issue_number_goes_here> 🦕

[gemini-code-assist]

Code Review

This pull request migrates Gemini model references from version 2.0 to 2.5 across several notebooks and system tests, while also resetting notebook execution counts. The review feedback identifies inconsistencies in model naming conventions (versioned vs. unversioned) and suggests expanding test coverage for the gemini-2.5-pro model, particularly where test function names imply its inclusion.

[gemini-code-assist]

There is an inconsistency in model naming conventions within this file. This test uses unversioned aliases (e.g., gemini-2.5-flash), while other tests (e.g., test_llm_gemini_score at line 169) use specific versioned snapshots (e.g., gemini-2.5-flash-001). It is recommended to use a consistent naming scheme across the test suite to ensure clarity and reproducibility. This applies to the parametrizations in the subsequent tests (Diffs 2-5) as well.

        "gemini-2.5-pro",
        "gemini-2.5-flash",
        "gemini-2.5-flash-lite",
        "gemini-2.5-flash-001",
        "gemini-2.5-flash-lite-001"

[gemini-code-assist]

This test is missing coverage for the gemini-2.5-pro model, which is included in other LLM tests in this file. Adding it here would ensure consistent validation of the scoring functionality across all supported Gemini models.

        "gemini-2.5-pro",
        "gemini-2.5-flash-001",
        "gemini-2.5-flash-lite-001",

[gemini-code-assist]

The function name test_llm_gemini_pro_score_params explicitly mentions pro, but the parametrization currently only includes flash models. This is misleading and suggests missing test coverage for the Pro model. Adding gemini-2.5-pro would align the test with its name and improve coverage.

        "gemini-2.5-pro",
        "gemini-2.5-flash-001",
        "gemini-2.5-flash-lite-001",