feat: add Substitution Cipher algorithm and tests

[mmohamedkhaled]

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Describe your change:

• Add an algorithm

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 JavaScript files are placed inside an existing directory.
• All filenames use the UpperCamelCase (PascalCase) style. There are no spaces in filenames.
• All new algorithms have a URL in their comments that points to Wikipedia or another similar explanation.
• All code is tested using Vitest and follows the ES Module syntax.
• Code is formatted using Prettier.

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Summary:

This PR adds a monoalphabetic Substitution Cipher to the ciphers/ directory.

• Includes substitutionCipherEncryption and substitutionCipherDecryption functions
• Validates key length and character set
• Test coverage includes:
  • Encryption
  • Decryption
  • Non-alphabetic character handling
  • Invalid key rejection
• All tests are passing using Vitest
• Code formatted with Prettier

📚 Reference: Wikipedia – Substitution Cipher

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.91%. Comparing base (1d252d7) to head (25add22).

Additional details and impacted files

@@ Coverage Diff @@ ## master #1773 +/- ## ========================================== + Coverage 85.87% 85.91% +0.04%  ========================================== Files 377 378 +1 Lines 19720 19779 +59 Branches 3005 3017 +12 ========================================== + Hits 16935 16994 +59  Misses 2785 2785 

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[mmohamedkhaled]

This PR only introduces the Substitution Cipher and its test. I’ve noticed the CI flags code style/spelling issues in files like:

String/AlphaNumericPalindrome.js

Recursive/SubsequenceRecursive.js

Maths/MobiusFunction.js

These issues are pre-existing in the repository and not introduced by this PR. I have run Prettier on my changes, and there are no further modifications I can make to fix these unrelated files.