[hangman] Sync tests & Update solution to match canonical rules

[thibault2705]

pull request

What has been done:

• Rename and simplify method play(Observable<String> words, Observable<String> letters) to guess(String word, Observable<String> letters)
• Add, rename output params maskedWord, state, and remainingFailures to match canonical expectations
• Introduced MAX_FAILURES = 10 based on canonical rules
• Added error handling for guesses after win and lose states
• Aligned test cases with canonical scenarios

This PR partially solves issues #2959.

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