Fix draft mode invariant (#62121)

### What?

In today's implementation, requests that don't end in `.rsc` are getting
the data request query. We need to remove this toggle to prevent them
from 500'ing.

### Why?

The invariant triggered was:


212553958c/packages/next/src/server/base-server.ts (L2838-L2842)

Crawling up the code, the conditional:


212553958c/packages/next/src/server/base-server.ts (L2815)

must be `true`.

The variable `isDataReq` is set here:


212553958c/packages/next/src/server/base-server.ts (L1833-L1839)

This conditional expression is a bit complex, but it simplifies down to:
- `isDataReq` is `true` when (TODO)
- `isDataReq` is `false` when (TODO)

### How?

Closes NEXT-2341
Fixes #61377

Tested manually by:
- Building and packaging this branch locally
- Deploying tarball and then depending on it as the Next.js version for
an example app similar to the one reported by #61377
- Inspecting console for 500 errors.

---------

Co-authored-by: JJ Kasper <jj@jjsweb.site>
Co-authored-by: Zack Tanner <zacktanner@gmail.com>

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

@@ -2831,7 +2831,11 @@ export default abstract class Server<ServerOptions extends Options = Options> {
         res.setHeader(NEXT_DID_POSTPONE_HEADER, '1')
       }
 
-      if (isDataReq) {
+      // we don't go through this block when preview mode is true
+      // as preview mode is a dynamic request (bypasses cache) and doesn't
+      // generate both HTML and payloads in the same request so continue to just
+      // return the generated payload
+      if (isDataReq && !isPreviewMode) {
         // If this is a dynamic RSC request, then stream the response.
         if (isDynamicRSCRequest) {
           if (cachedData.pageData) {