Defines handlers that only execute when the user is in a specific FSM state.

See Finite State Machines for a complete multi-step example.

@spec get_state(atom(), chat_id()) :: {term(), term()}

Retrieves the current FSM state and data for a user.

Returns {nil, nil} when the chat has no stored state.

@spec init(atom()) :: :ok | {:error, term()}

Initializes the FSM storage for a bot.

This is called automatically by TelegramEx.Server when the bot starts.

@spec reset_state(atom(), chat_id()) :: atom() | {:error, term()}

Resets the FSM state for a user, removing their stored state and data.

@spec set_state(atom(), chat_id(), atom()) :: atom() | {:error, term()}

Sets the FSM state for a user, keeping existing data.

@spec set_state(atom(), chat_id(), atom(), term()) :: atom() | {:error, term()}

Sets the FSM state and data for a user.