Preserve shared FIAS check-ins and assign guest groups

[Stell0]

FIAS Guest Check-In handling does not correctly preserve an already occupied shared room. The optional GG guest group number should also be mapped to NethHotel room groups.

Steps to reproduce

1. Configure FIAS GI processing to include GG.
2. Check in room 201 with reservation R1, guest Alice Example, GS=Y, and GG=GROUP-1.
3. Check in the same room with reservation R2, guest Bob Example With Long Surname, GS=Y, and GG=GROUP-1.
4. Check in room 202 with GG=GROUP-1.
5. Verify the occupancy, guest label, reservations, and room group assignments.

Expected behavior

Room 201 remains checked in with both guests and a complete shared guest label. Group GROUP-1 is created or reused and rooms 201 and 202 are assigned to it.

Actual behavior

Adding a shared guest performs a new full check-in, which can force the existing room occupancy through a checkout/check-in cycle. The room guest label is limited to 32 characters, and the GG field is ignored.