Add comprehensive doctests to arc_length.py

[Tejasrahane]

• Improved type hints (int -> float)
• Added edge case tests (angle=0, 360, 180)
• Added float value tests
• Added error handling for negative values
• Enhanced docstring with formula and Wikipedia link
• Added proper error messages

Contributes to #9943

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.
• 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".

[Tejasrahane]

Great improvements to the arc_length.py file! Here's my review:

Strengths:

1. Comprehensive Doctests: Excellent addition of edge case tests (angle=0, 360, 180) and error handling tests for negative values.

2. Improved Type Hints: Changing from int to float in the function signature is correct since arc length calculations typically work with floating-point numbers.

3. Better Documentation: The enhanced docstring with formula, Wikipedia link, and clear parameter descriptions makes the function much more understandable.

4. Proper Error Handling: Adding ValueError for negative inputs with descriptive messages is a solid improvement.

Suggestions for Improvement:

1. Doctest Import Location: Consider moving import doctest to the top of the file with other imports rather than at the bottom, following PEP 8 conventions.

2. Float Value Tests: Some of the doctest examples use float inputs (e.g., arc_length(45.5, 10.5)), which is great for testing, but the expected output values should be verified for accuracy.

3. Consider Testing Larger Angles: While you test 0, 180, and 360, it might be worth noting in the docstring whether angles > 360 are expected or should also raise an error.

Overall, this is a quality PR that significantly improves the code quality and testing coverage. Nice work!