Add Java Solution for Smallest Repunit Divisible by K

[ArsiyaTasleem90250]

This PR adds a Java solution to find the smallest repunit (a number consisting of only 1s) that is divisible by a given integer K.

Key features:

• Efficiently calculates the smallest number made of repeated 1s divisible by K.
• Handles edge cases where no such repunit exists.
• Demonstrates the use of modular arithmetic to avoid integer overflow.
• Can be used as a reference implementation for numeric problems involving divisibility and patterns.

This contribution is part of Hacktoberfest 2025 participation and adds a new problem-solving example to the repository.