Fixes holoparasite injector being usable on anything

Content.Server/Guardian/CanHostGuardianComponent.cs

@@ -0,0 +1,8 @@
+using Robust.Shared.GameObjects;
+
+namespace Content.Server.Guardian;
+
+[RegisterComponent, ComponentProtoName("CanHostGuardian")]
+public class CanHostGuardianComponent : Component
+{
+}

Content.Server/Guardian/GuardianSystem.cs

@@ -126,6 +126,14 @@ namespace Content.Server.Guardian
                 return;
             }
 
+            // Can only inject things with the component...
+            if (!HasComp<CanHostGuardianComponent>(target))
+            {
+                _popupSystem.PopupEntity(Loc.GetString("guardian-activator-invalid-target"), user, Filter.Entities(user));
+                return;
+            }
+
+
             // If user is already a host don't duplicate.
             if (HasComp<GuardianHostComponent>(target))
             {