Add gcd(0, 0) edge case to doctests

[yaadhuu]

Describe your change:

• Fix a bug or typo in an existing algorithm?

This pull request handles the edge case where both inputs to the
Greatest Common Divisor (GCD) function are zero (gcd(0, 0)), which was
previously undefined. Doctests have been added to document and verify
the expected behavior.

Checklist:

• I have read 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, separate PRs are used 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.
• All functions have doctests that pass the automated testing.