[unstable_after] fix occasional dev-mode crash in edge runtime pages (#…

…66099)

When hot-reloading an edge runtime page, sometimes we randomly get this:
```
[InvariantError: Invariant: Cannot call onClose on a response that is already sent. This is a bug in Next.js.] {
  name: 'InvariantError'
}
```
and since `unstable_after` relies on `onClose`, no callbacks are called.

I believe this is because `web-server` [calls `res.send()` pretty early
for streaming
responses](https://github.com/vercel/next.js/blob/b9817f83519fdaa7a7fcd4711dfb80aa1e1a2434/packages/next/src/server/web-server.ts#L311),
so `res.sent` becomes `true` even though we're still streaming, and
prevents us from calling `onClose` in the streaming render. It makes
more sense to check the CloseController's `isClosed` instead -- when
that becomes `true`, we _really_ shouldn't call onClose anymore (points
to a bug), but before that it's fine.

packages/next/src/server/base-http/web.ts

@@ -137,9 +137,9 @@ export class WebNextResponse extends BaseNextResponse<WritableStream> {
         'Cannot call onClose on a WebNextResponse initialized with `trackOnClose = false`'
       )
     }
-    if (this.sent) {
+    if (this.closeController.isClosed) {
       throw new InvariantError(
-        'Cannot call onClose on a response that is already sent'
+        'Cannot call onClose on a WebNextResponse that is already closed'
       )
     }
     return this.closeController.onClose(callback)