Address feedback: Combine tests using pytest.mark.parametrize

[Copilot]

Description

Combines two separate tests for contributions_over_time data parameter handling into a single parametrized test using pytest.mark.parametrize.

Before:

def test_contributions_over_time_accepts_data_parameter(mock_posterior_data):
    ...

def test_contributions_over_time_data_parameter_fallback(mock_idata_with_posterior):
    ...

After:

@pytest.mark.parametrize(
    "use_explicit_data",
    [
        pytest.param(True, id="explicit_data_parameter"),
        pytest.param(False, id="fallback_to_idata"),
    ],
)
def test_contributions_over_time_data_parameter(
    use_explicit_data, mock_posterior_data, mock_idata_with_posterior
):
    ...

Related Issue

• Closes #
• Related to MMM Plot Suite Migration: arviz_plots Multi-Backend Support #2098

Checklist

• Checked that the pre-commit linting/style checks pass. Feel free to comment pre-commit.ci autofix to auto-fix.
• Included tests that prove the fix is effective or that the new feature works
• Added necessary documentation (docstrings and/or example notebooks) using numpydoc format.
• If you are a pro: each commit corresponds to a relevant logical change

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