Add comprehensive doctests for maths/greatest_common_divisor.py

[Achintya47]

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?

This PR adds comprehensive doctests to maths/greatest_common_divisor.py to improve test coverage from 47% to near 100%. The enhanced doctests cover:

• Basic GCD calculations with various integer pairs
• Edge cases with zero values (gcd(0,n) and gcd(n,0))
• Negative number handling in all combinations
• Large number testing to verify algorithm scalability
• Verification that both recursive and iterative implementations produce identical results
• Cases where numbers are coprime, identical, or one divides the other

All doctests have been independently verified for correctness and pass when run locally.

Contributes to #9943

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, 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.
• All functions have doctests that pass the automated testing.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• If this pull request resolves one or more open issues then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".