Add solution for Longest Substring Without Repeating Characters

[Piyush-Kumar62]

Added a Java solution for finding the longest substring without repeating characters.

• Uses sliding window + HashMap and ASCII optimization.
• Returns both length and one example substring.
• Handles edge cases like empty strings and repeated chars.
• Tested with multiple sample inputs.

Folder: Java/Strings
Language: Java
Hacktoberfest ready ✅