Add _rsc query param to earliest RSC request detection
The earliest RSC detection now checks both the `rsc` header AND the
`_rsc` URL search param. This handles CDN/proxy scenarios where the
`rsc` header may be stripped but the `_rsc` cache-busting param in
the URL is preserved.
Changes:
- adapter.ts: isRSCRequest is now true when either the `rsc` header is
set or `_rsc` is present in the URL
- base-server.ts handleRSCRequest: also detects RSC via `_rsc` query
param; sets `rsc` header for downstream compatibility when needed
- resolve-routes.ts: RSC rewrite headers are now set for requests with
`_rsc` in the URL, even without the `rsc` header
Co-authored-by: shuding <3676859+shuding@users.noreply.github.com>
