Fix/prefetch always 200

[denesbeck]

Fixes: #85121

Starting from Next.js v15.4.1 (commit 256cd5b571), prefetch requests for SSG/ISR pages always returned 200 OK status codes instead of 304 Not Modified, even when the content hadn't changed. This forced full payload downloads on every prefetch (e.g., on hover or in-viewport), unnecessarily increasing CDN traffic and bandwidth costs.

Root Cause:
In commit 256cd5b571 ("Add response handling inside handlers"), the Pages Router handler was modified to create RenderResult instances with Buffer.from() for cached content:

The problem: RenderResult.isDynamic returns true when the response is a Buffer (not a string):

public get isDynamic(): boolean {
  return typeof this.response !== 'string'  // Buffer !== 'string' → true
}

Test repository for testing changes (to prevent version conflicts): https://github.com/denesbeck/next-test-304
Simply package the Next.js version from this branch into the repository. Then, run npm run build and npm run start.

Note: Firefox may show different behavior as it handles prefetch caching with different strategies that don't always use the same HTTP cache validation mechanisms as Chrome. However, the server-side fix is correct and working as expected - the server properly generates ETags and returns 304 responses when appropriate. The difference is in how browsers choose to send (or not send) the If-None-Match header for prefetch requests.

[ijjk]

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