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

@@ -1,5 +1,5 @@
 params [ "_minimum_readiness", "_is_infantry" ];
-private [ "_headless_client" ];
+private [ "_headless_client", "_grp", "_spawn_marker", "_sector_spawn_pos", "_started_time", "_patrol_continue" ];
 
 waitUntil { !isNil "blufor_sectors" };
 waitUntil { !isNil "combat_readiness" };
@@ -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;
@@ -58,6 +62,11 @@ while { GRLIB_endgame == 0 } do {
     };
 
     while { _patrol_continue } do {
+        if (isNil "_grp") then {
+            _patrol_continue = false;
+            ["_grp was nil, but patrol was expected", "MANAGE_ONE_PATROL"] call KPLIB_fnc_log;
+            break;
+        };
         sleep 60;
         if ( count (units _grp) == 0  ) then {
             _patrol_continue = false;
@@ -76,6 +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);
     };