Add rejectDraw method to GameManager

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Feature Request\n\nThe handles draw offers via and , but there's no way for a player to explicitly reject a draw offer.\n\nCurrent behavior:\n- Player A offers a draw ()\n- is set to A's color\n- Player B can accept it ()\n- If B wants to decline, they must either ignore it or make a move (which doesn't clear the offer)\n\nProposed behavior:\nAdd a method that:\n1. Returns if the player isn't in a game or no draw is outstanding\n2. Clears when called\n3. Returns the updated game\n\nThis keeps the draw offer protocol more symmetrical and gives explicit feedback to the client.\n\n\nFiles likely affected:\n- — add the method\n- — add to interface\n- — add tests