`reconnect()` can produce partial resubscriptions

In `reconnect()`, subscriptions are re-established:
https://github.com/adafruit/Adafruit_CircuitPython_MiniMQTT/blob/bf47a0e613235a97346ab0f7b20d727d666c2227/adafruit_minimqtt/adafruit_minimqtt.py#L970-L975

But there's a timing issue here; if the above code fails partway through re-subscribing, there's a silent failure case; witness:

```python
#
mqtt_client.subscribe("foo")
mqtt_client.subscribe("bar")

while True:
try:
mqtt_client.loop()
except MMQTTException:
while True:
mqtt_client.reconnect()
break
except MMQTTException:
# reconnect failed; wait a bit and try again
time.sleep(5)
```

In this scenario, if a reconnect happens and `subscribe("foo")` succeeds but "bar" throws an exception, then `_subscribed_topics` will merrily report `["foo"]`, and the next reconnection attempt will subscribe to "foo" and consider its job complete, returning control to the main loop without ever reattempting "bar".

(Moved from its original home in #252, with h/t to [this comment](https://github.com/adafruit/Adafruit_CircuitPython_MiniMQTT/issues/252#issuecomment-3872224698).)

Start with reconnect() in adafruit_minimqtt/adafruit_minimqtt.py at the linked subscription-resubscription code, and trace how _subscribed_topics changes when subscribe("foo") succeeds but subscribe("bar") raises MMQTTException. Reproduce that sequence from the issue; done means a later reconnect retries every subscription that was not successfully restored, rather than treating the partial result as complete.

Written by the indexing model from the issue text.