chore: final verification fixes for sitemap reliability

- Replace invalid revalidate export with unstable_cache in sitemap.ts
- Add revalidateTag('sitemap', 'default') to on-demand revalidation endpoint
- Fix test assertions to match /game/ URL path

app/api/revalidate-sitemap/route.ts

@@ -1,4 +1,4 @@
-import { revalidatePath } from "next/cache"
+import { revalidatePath, revalidateTag } from "next/cache"
 import { NextRequest, NextResponse } from "next/server"
 
 /**
@@ -38,6 +38,7 @@ export async function POST(request: NextRequest): Promise<NextResponse> {
   }
 
   try {
+    revalidateTag("sitemap", "default")
     revalidatePath("/sitemap.xml")
     return NextResponse.json({
       revalidated: true,