Fix RequestHandler loadContext type when middleware is enabled (#13204)

Author: brophdawg11

.changeset/few-months-begin.md

@@ -0,0 +1,5 @@
+---
+"react-router": patch
+---
+
+Fix `RequestHandler` `loadContext` parameter type when middleware is enabled

packages/react-router/lib/server-runtime/server.ts

@@ -41,7 +41,7 @@ import type { MiddlewareEnabled } from "../types/future";
 export type RequestHandler = (
   request: Request,
   loadContext?: MiddlewareEnabled extends true
-    ? unstable_RouterContextProvider
+    ? unstable_InitialContext
     : AppLoadContext
 ) => Promise<Response>;