Add PermutationCipher Implementation

Description

This PR adds a newPermutationCipher 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: DescriptiveIllegalArgumentException 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