fix: fixing return RPC not generating serialize functions

if only using a type in a UniTask<MyType> RpcMyRpc(), a serialize functions for MyType will now be generated. previous this would give runtime error because no writer for MyType found.

Assets/Mirage/Weaver/Processors/RpcProcessor.cs

@@ -305,7 +305,15 @@ protected ReturnType ValidateReturnType(MethodDefinition md, RemoteCallType call
             // UniTask is allowed
             var unitaskType = typeof(UniTask<int>).GetGenericTypeDefinition();
             if (returnType.Is(unitaskType))
+            {
+                var genericReturnType = (GenericInstanceType)returnType;
+                var genericArg = genericReturnType.GenericArguments[0];
+                // ensure serialize functions exist
+                _ = writers.GetFunction_Throws(genericArg);
+                _ = readers.GetFunction_Throws(genericArg);
+
                 return ReturnType.UniTask;
+            }
 
             throw new RpcException($"Use UniTask<{md.ReturnType}> to return values from [ClientRpc] or [ServerRpc]", md);
         }