feat(ciphers): Add PermutationCipher implementation(enhance #6697)

[dheeraj1010]

Add PermutationCipher Implementation

Description

This PR adds a new PermutationCipher class to the ciphers package, implementing a transposition cipher that encrypts text by rearranging characters according to a fixed permutation key.

Algorithm Overview

The Permutation Cipher is a classic transposition cipher that:

1. Divides plaintext into blocks of key length
2. Rearranges characters within each block according to key positions
3. Pads incomplete blocks with 'X' characters
4. Supports bidirectional encryption/decryption

Example: With key {3, 1, 2} and plaintext "HELLO":

• Block 1: "HEL" → positions {3,1,2} → "LHE"
• Block 2: "LOX" → positions {3,1,2} → "XLO"
• Result: "LHEXLO"

Implementation Features

✅ Core Functionality

• encrypt(String, int[]) - Encrypts plaintext with permutation key
• decrypt(String, int[]) - Decrypts ciphertext using inverse permutation
• Automatic padding for incomplete blocks using 'X' character
• Text preprocessing (removes spaces, converts to uppercase)

✅ Robust Validation

• Key validation: Ensures 1-based positions without duplicates
• Range checking: Validates all positions are within key length
• Error handling: Descriptive IllegalArgumentException messages
• Null safety: Handles null inputs gracefully

✅ Comprehensive Testing

• 20+ JUnit 5 test cases with full coverage
• Round-trip testing (encrypt → decrypt → verify)
• Edge cases: empty strings, single char keys, exact block sizes
• Error scenarios: invalid keys, duplicates, out-of-range values
• Real-world examples: secret messages, longer texts, reverse permutations

Files Added

• src/main/java/com/thealgorithms/ciphers/PermutationCipher.java - Main implementation
• src/test/java/com/thealgorithms/ciphers/PermutationCipherTest.java - Comprehensive test suite

Code Quality

• ✅ JavaDoc documentation with algorithm explanation and examples
• ✅ Follows project conventions matching existing cipher implementations
• ✅ Clean, readable code with meaningful variable names
• ✅ Comprehensive error handling with descriptive messages
• ✅ Efficient implementation with O(n) time complexity

Testing Results

[codecov-commenter]

Codecov Report

❌ Patch coverage is 94.54545% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.93%. Comparing base (74647e3) to head (246a449).

Files with missing lines	Patch %	Lines
...a/com/thealgorithms/ciphers/PermutationCipher.java	94.54%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6700      +/-   ##
============================================
+ Coverage     75.88%   75.93%   +0.04%     
- Complexity     5819     5845      +26     
============================================
  Files           706      707       +1     
  Lines         19910    19965      +55     
  Branches       3859     3871      +12     
============================================
+ Hits          15109    15160      +51     
- Misses         4218     4219       +1     
- Partials        583      586       +3     

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[dheeraj1010]

Please review @DenizAltunkapan @yanglbme @alxkm

[alxkm]

Looks good, thanks.

[dheeraj1010]

Thanks a lot @alxkm .