Refactor binary to decimal conversion function

[Ewanjohndennis]

Rewrote bin_to_decimal to use Python's built-in int(s, 2) instead of a manual loop that accumulates the decimal value digit by digit. The logic and doctests are unchanged. Also added a guard for "-" as input, which previously slipped past the empty-string check and raised an unhelpful ValueError about non-binary values.

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• Fix a bug or typo in an existing algorithm?
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