Added One-Time Pad Cipher implementation in Java #6941

[Shewale41]

# ✨ Add One-Time Pad Cipher Implementation  
Fixes #6941  

This PR adds a simple implementation of the **One-Time Pad Cipher** in Java under the `ciphers` package. It demonstrates XOR-based encryption and decryption using a random key of equal length to the plaintext.  

### 🧩 File Added  
`src/main/java/com/thealgorithms/ciphers/OneTimePadCipher.java`  

### ⚙️ Implementation Details  
- Uses `SecureRandom` for key generation  
- Encrypts and decrypts using the same XOR logic  
- Includes a small demo in the `main()` method  
- Time Complexity: **O(n)**  
- Space Complexity: **O(n)**  

### 🧪 Example Usage  
**Input:**  

Message: HELLO WORLD
Key: 4@_Zb1+|hG#

**Output:**  

Encrypted: �$%&!
Decrypted: HELLO WORLD

### 🧠 Summary  
This algorithm showcases the concept of perfect secrecy through XOR operation, where encryption and decryption use the same key. It’s lightweight, symmetric, and educational for understanding cryptographic basics.  

### 🧑‍💻 Author  
[@Shewale41](https://github.com/Shewale41)  

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 22 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.19%. Comparing base (21eff8a) to head (a179b5b).

Files with missing lines	Patch %	Lines
...va/com/thealgorithms/ciphers/OneTimePadCipher.java	0.00%	22 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6965      +/-   ##
============================================
- Coverage     78.26%   78.19%   -0.08%     
- Complexity     6573     6574       +1     
============================================
  Files           743      744       +1     
  Lines         21895    21917      +22     
  Branches       4288     4290       +2     
============================================
+ Hits          17137    17138       +1     
- Misses         4074     4096      +22     
+ Partials        684      683       -1     

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