fix: prevent group_diag from prematurely marking patrol groups for deletion

Tags patrol groups (KPLIB_patrol_group) so group_diag skips them, and
cleans up the tag + enables deleteGroupWhenEmpty only once the patrol
lifecycle is complete. Prevents the '_grp was nil' log spam by fixing
the root cause rather than just silencin the symptom.

src/scripts/server/offloading/group_diag.sqf

@@ -12,6 +12,7 @@ while {true} do {
         };
 
         if !(isNil "_owner") then {
+            if (_x getVariable ["KPLIB_patrol_group", false]) then {continue;};
             if !(isGroupDeletedWhenEmpty _x) then {
                 if (local _x) then {
                     _x deleteGroupWhenEmpty true;

src/scripts/server/patrols/manage_one_patrol.sqf

@@ -45,6 +45,10 @@ while { GRLIB_endgame == 0 } do {
         _grp = group ((crew _vehicle_object) select 0);
     };
 
+    if (!isNull _grp) then {
+        _grp setVariable ["KPLIB_patrol_group", true, true];
+    };
+
     [_grp] spawn patrol_ai;
 
     _started_time = time;
@@ -81,6 +85,15 @@ while { GRLIB_endgame == 0 } do {
         };
     };
 
+    if (!isNull _grp) then {
+        _grp setVariable ["KPLIB_patrol_group", nil, true];
+        if (local _grp) then {
+            _grp deleteGroupWhenEmpty true;
+        } else {
+            [_grp, true] remoteExecCall ["deleteGroupWhenEmpty", groupOwner _grp];
+        };
+    };
+
     if ( !([] call KPLIB_fnc_isBigtownActive) ) then {
         sleep (600.0 / GRLIB_difficulty_modifier);
     };