Allow converting numbers in heat circuit names

[kysel]

This small PR adds ability to correctly convert numbers using CHAR_MAP constant.
Number can be used in heating circuit names and before this change it would error out like the traceback below.
After this change the library is able to correctly decode circuit names even with numbers

Traceback (most recent call last):
  File "/usr/src/homeassistant/homeassistant/helpers/update_coordinator.py", line 419, in _async_refresh
    self.data = await self._async_update_data()
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/config/custom_components/mastertherm/coordinator.py", line 156, in _async_update_data
    refreshed = await self.mt_controller.refresh()
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.13/site-packages/masterthermconnect/controller.py", line 394, in refresh
    device["data"] = self.__populate_data(
                     ~~~~~~~~~~~~~~~~~~~~^
        DEVICE_READ_MAP, device["api_full_data"]
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "/usr/local/lib/python3.13/site-packages/masterthermconnect/controller.py", line 120, in __populate_data
    data[key] = self.__populate_data(device_map[key], registers)
                ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.13/site-packages/masterthermconnect/controller.py", line 120, in __populate_data
    data[key] = self.__populate_data(device_map[key], registers)
                ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.13/site-packages/masterthermconnect/controller.py", line 118, in __populate_data
    data[key] = self.__convert_data(item[0], item[1], registers)
                ~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.13/site-packages/masterthermconnect/controller.py", line 91, in __convert_data
    item_str = item_str + CHAR_MAP[int(registers[list_value])]
                          ~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^
IndexError: list index out of range