The VT formatter was treating cells without text as blank and emitting
them as plain spaces, losing any background color styling. This caused
TUIs like htop to lose their background colors when rehydrating terminal
state (e.g., after detach/reattach in zmx).
For styled formats (VT/HTML), cells with background colors or style_id
are now emitted with proper SGR sequences and a space character instead
of being accumulated as unstyled blanks.
Adds handling for bg_color_palette and bg_color_rgb content tags which
were previously unreachable.
Reference: https://ampcode.com/threads/T-019b7a35-c3f3-73fc-adfa-00bbe9dbda3c
6fdbc29b9a