Error with Pico & ESP-01

When interfacing Raspberry Pico to ESP-01 via UART, received
```
Traceback (most recent call last):
File "", line 167, in
File "", line 129, in main
File "", line 96, in esp_test
File "/lib/adafruit_espatcontrol/adafruit_espatcontrol.py", line 357, in is_connected
File "/lib/adafruit_espatcontrol/adafruit_espatcontrol.py", line 126, in begin
AttributeError: 'ESP_ATcontrol' object has no attribute 'cipmux'
```

If I remove the 2 lines (125 & 126) it seems to resolve the issue as a dirty fix. It appears that this particular implementation of the board refuses to use a CIPMUX of 0 just from probing with manual AT commands.

https://github.com/adafruit/Adafruit_CircuitPython_ESP_ATcontrol/blob/96ae96d6ad39204ca876a9a0cc0a35882dd00fa8/adafruit_espatcontrol/adafruit_espatcontrol.py#L126

Start by reading adafruit_espatcontrol/adafruit_espatcontrol.py around lines 125-126 and the begin/is_connected call path. Reproduce the UART interaction with a Raspberry Pico and ESP-01, checking how this board responds to CIPMUX. Done means the reported connection path no longer raises AttributeError for this hardware while preserving behavior for supported responses.

Written by the indexing model from the issue text.