Fix issue when a cast stereo pair is added to a cast group (#2480)

Fix issue when a cast stereo pair is added to a cast group (#2480)

* Update helpers.py

Fix: for Chromecast group with stereo pair and another speaker becomes unavailable https://github.com/music-assistant/support/issues/4106

* Update helpers.py

Update for linting

* Fix: Update helpers.py

typo correction

* Update music_assistant/providers/chromecast/helpers.py

Co-authored-by: OzGav <gavnosp@hotmail.com>
* Update helpers.py

Added code comments

* Update helpers.py comment added

line break to pass linting

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Co-authored-by: OzGav <gavnosp@hotmail.com>
Co-authored-by: Marcel van der Veldt <m.vanderveldt@outlook.com>