Prevent invalid utf8 indexing in cell magic detection

[konstin]

The example below used to panic because we tried to split at 2 bytes in the 4-bytes character 转.

def sample_func(xx):
    """
    转置 (transpose)
    """
    return xx.T

Fixes #9145
Fixes astral-sh/ruff-vscode#362

The second commit is a small test refactoring.

[github-actions]

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[dhruvmanila]

Thanks!