Add water droplet hat for arachnids (#23822)

Co-authored-by: Kevin Zheng <kevinz5000@gmail.com>

Content.Server/Chemistry/ReagentEffects/WearableReaction.cs

@@ -0,0 +1,42 @@
+using Content.Shared.Inventory;
+using Content.Shared.Chemistry.Reagent;
+using Robust.Shared.Prototypes;
+
+namespace Content.Server.Chemistry.ReagentEffects;
+
+/// <summary>
+/// A reaction effect that consumes the required amount of reagent and spawns PrototypeID in the
+/// entity's Slot. Used to implement the water droplet effect for arachnids.
+/// </summary>
+public sealed partial class WearableReaction : ReagentEffect
+{
+    /// <summary>
+    /// Minimum quantity of reagent required to trigger this effect.
+    /// </summary>
+    [DataField]
+    public float AmountThreshold = 1f;
+
+    /// <summary>
+    /// Slot to spawn the item into.
+    /// </summary>
+    [DataField(required: true)]
+    public string Slot;
+
+    /// <summary>
+    /// Prototype ID of item to spawn.
+    /// </summary>
+    [DataField(required: true)]
+    public string PrototypeID;
+
+    protected override string? ReagentEffectGuidebookText(IPrototypeManager prototype, IEntitySystemManager entSys) => null;
+
+    public override void Effect(ReagentEffectArgs args)
+    {
+        if (args.Reagent == null || args.Quantity < AmountThreshold)
+            return;
+
+        // SpawnItemInSlot returns false if slot is already occupied
+        if (args.EntityManager.System<InventorySystem>().SpawnItemInSlot(args.SolutionEntity, Slot, PrototypeID))
+            args.Source?.RemoveReagent(args.Reagent.ID, AmountThreshold);
+    }
+}