Fixed ComputeRosterType to correctly handle unassigned players

[tintinhamans]

• Fixed an issue where matches consisting only of unassigned players (team -1) would result in empty strings (e.g. 1v1s).
• Updated grouping logic to append unassigned players as individual groups of 1 using Enumerable.Repeat, eliminating the standalone FFA check.
• Updated to return "Unknown" for matches where all active players are on the same team (groups.Count == 1).