Added Johnson's algorithm for all-pairs shortest paths

[sangampaudel530]

Describe your change:

This PR adds Johnson's algorithm for computing all-pairs shortest paths in a weighted graph. The implementation handles graphs with negative edge weights (but no negative cycles) by combining Bellman-Ford and Dijkstra algorithms. A corresponding test file test_johnson.py is also included to validate correctness.

• 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: Johnson's algorithm
• 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".

[algorithms-keeper]

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🔗 Relevant Links

Repository:

• Contributing guidelines
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Python:

• Formatted string literals (f-strings)
• Type hints
• doctest
• unittest
• pytest

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[sangampaudel530]

Hi @cclauss @MaximSmolskiy Could you review this pr and give some feedback. Could you please merge this after reviewing it ?

[sangampaudel530]

Hi @cclauss , I wanted to kindly ask if there’s any feedback or required changes for this PR.
All checks have passed, and I’d be happy to make any improvements needed to help with review.
Thanks for your time and for maintaining this great project!