[TASK] Unit Tests for Hangman game

[jody]

Prerequisite: #85

Task

Add JUnit tests for the HangmanGame class.

Consider the guidance from the parent issue (#84):

Simulate winning a game where the user guesses letters and wins.
Simulate a winning game by a full word guess.
Simulate a losing game after 6 incorrect guesses.
Simulate repeated letter guesses to ensure they don't reduce remaining tries
Simulate invalid input (non-alphabetic) and ensure it is igrnored

Create test methods in HangmanGameTest.java

Verify that each test runs properly, and gives appropriate results.

[Mohamed-47]

@Mohamed-47
I am interested in this issue