fix new GetFromNamespacedId method not handling prefix IDs correctly

docs/release-notes.md

@@ -4,6 +4,8 @@
 ## Upcoming release
 * For players:
   * Improved performance when mods edit maps (thanks to SinZ!).
+* For mod authors:
+  * Fixed new `helper.ModRegistry.GetFromNamespacedId` not handling prefix IDs correctly.
 
 ## 4.3.0
 Released 12 July 2025 for Stardew Valley 1.6.14 or later. See [release highlights](https://www.patreon.com/posts/133992196).

src/SMAPI/Framework/ModHelpers/ModRegistryHelper.cs

@@ -85,7 +85,7 @@ internal class ModRegistryHelper : BaseHelper, IModRegistry
             }
         }
 
-        return null;
+        return mod;
     }
 
     /// <inheritdoc />