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.

Describe your change:

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• Documentation change?

Checklist:

• I have read [CONTRIBUTING.md](https://github.com/TheAlgorithms/Python/blob/master/CONTRIBUTING.md).
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python [type hints](https://docs.python.org/3/library/typing.html).
• All functions have [doctests](https://docs.python.org/3/library/doctest.html) that pass the automated testing.
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• If this pull request resolves one or more open issues then the description above includes the issue number(s) with a [closing keyword](https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue): "Fixes #ISSUE-NUMBER". (no linked issue)

[Ewanjohndennis]

Hi, thanks for the review!

Sorry for the ping, but since this PR has been approved, could a maintainer please take a look and merge if everything looks good?

@cclauss @MaximSmolskiy