getBlitzContext unusable in api

[gengjiawen]

This will result in build failed.

Attempted import error: 'setupBlitzServer' is not exported from '@blitzjs/next' (imported as 'setupBlitzServer').

Originally posted by @gengjiawen in #4303 (comment)

Minimal repro: gengjiawen/blitzjs-demo@064bdde

direct debug link: https://gitpod.io/?autostart=true#https://github.com/gengjiawen/blitzjs-demo/commit/064bddecd9c0ba6e4fbd3bbb7d0a7944b3eea456

[gengjiawen]

@siddhsuresh @flybayer

[gengjiawen]

I figure it out on my own, turns out this can be used only on app router api, not available on page router api.