Fix vent critters spawn (#17833)

* VentCrittersRule scaling chance + spawn on station

* merge mouse migration and vent critters

* fix prototypes

* check if there is no valid locations

Content.Server/StationEvents/Components/MouseMigrationRuleComponent.cs

@@ -1,16 +0,0 @@
-using Content.Server.StationEvents.Events;
-
-namespace Content.Server.StationEvents.Components;
-
-[RegisterComponent, Access(typeof(MouseMigrationRule))]
-public sealed class MouseMigrationRuleComponent : Component
-{
-    [DataField("spawnedPrototypeChoices")]
-    public List<string> SpawnedPrototypeChoices = new() //we double up for that ez fake probability
-    {
-        "MobMouse",
-        "MobMouse1",
-        "MobMouse2",
-        "MobRatServant"
-    };
-}

Content.Server/StationEvents/Components/VentCrittersRuleComponent.cs

@@ -8,4 +8,10 @@ public sealed class VentCrittersRuleComponent : Component
 {
     [DataField("entries")]
     public List<EntitySpawnEntry> Entries = new();
+
+    /// <summary>
+    /// At least one special entry is guaranteed to spawn
+    /// </summary>
+    [DataField("specialEntries")]
+    public List<EntitySpawnEntry> SpecialEntries = new();
 }