Adds Longest Arithmetic Subsequence (Review Reqd)

[tejaswi0910]

Describe your change:

Summary:
This PR adds an implementation of the Longest Arithmetic Subsequence algorithm in Python. The function computes the length of the longest arithmetic subsequence from an input list of integers.

Key Features:
Implements the function longest_arithmetic_subsequence(nums: List[int]) -> int which:
Returns the length of the longest arithmetic subsequence.
Handles edge cases like empty arrays and None input.

Changes:
Added a function to calculate the longest arithmetic subsequence, supporting dynamic programming.
Added type hints using List from the typing module for clarity and static type checking.
The algorithm returns the length of the longest subsequence while ensuring:
For an empty array, the result is 0.
For a None array, a ValueError is raised.

• 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".

[imSanko]

Is it closed?

[tejaswi0910]

Is it closed?

I am working on it currently, no its not closed
its a draft PR in progress

[imSanko]

Can we add some more tests?

[tejaswi0910]

hey @tianyizheng02, can you please review this PR!