task: Add type hints to inversions.py

[nupurvavhal]

Describe your change:

Added type hints to count_inversions_bf, count_inversions_recursive, and helper functions for better code clarity and IDE support. Tested locally with python3 divide_and_conquer/inversions.py.

• 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".

[Copilot]

Pull request overview

Adds Python type hints to the inversion-counting implementation in divide_and_conquer/inversions.py to improve readability and editor/static-analysis support.

Changes:

• Annotated count_inversions_bf() parameter/return types.
• Annotated count_inversions_recursive() and _count_cross_inversions() parameter/return types.
• Added a return annotation to main().

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