Disable client-only for middleware and pages api layer (#55541)
We need to disable the default treat `middleware` and `pages/api` as
server-only, unless users explictly import "server-only" to poison it.
This will avoid the case that when a library is mixing "client-only" API
and shared components API in one bundle, and the shared API is used in
middleware or `pages/api` that might cause error. See the test case
added.
Follow up for #55394
