Reopens #93710 from a branch on this repo so the required deploy test
matrix can run — GitHub Actions doesn't expose secrets to fork PRs, and
those deploy jobs are required checks. All commits and credit carry
forward from the original PR by @elishagreenwald, which built on earlier
attempts by @LiamBoz in #84525 and @claygeo in #92053.

When middleware rewrites a Server Action POST to a page that doesn't
bundle the action, the receiving worker forwards the request to a worker
that does. The forwarded request hits middleware again and gets
rewritten the same way, so the new receiving worker forwards again —
looping until undici's headers timeout (~300s) or memory pressure brings
the server down (#84504). The fix is two related changes on the
forwarding code path. The first is an `!actionWasForwarded` guard in
`action-handler.ts` so a request carrying `x-action-forwarded: 1` is
never forwarded a second time. The second is a new branch in
`createForwardedActionResponse` that copies the
`x-nextjs-action-not-found` header and 404 status onto the originating
response when the forwarded worker can't find the action either; without
it the client would see a generic "unexpected response" error instead of
the `UnrecognizedActionError` that the rest of the framework expects.

The e2e test at `test/e2e/app-dir/action-forward-loop` reproduces the
scenario. A `proxy.ts` rewrites POSTs to `/with-action` so GETs still
render the page normally; the page renders a `<form>` with an inline
server action wrapped in a React error boundary that branches on
`unstable_isUnrecognizedActionError`. The test clicks the button and
waits for `#action-not-found-error`. Without the loop guard the test
times out at 10s, and without the pass-through the boundary catches a
generic error and the assertion fails — so both changes are individually
load-bearing.

This is a short-term fix. The mid-term direction is to remove
server-side action forwarding altogether and have the client dispatch
each Server Action to the route that actually bundles it, which makes
the entire forwarding code path (and this bug surface) unnecessary.
#90549 is the draft exploring that approach.

Fixes #84504

Closes #93710
Closes #84525
Closes #92053

---------

Co-Authored-By: Elisha Greenwald <ejgreenwald@gmail.com>
Co-Authored-By: LiamBoz <thisamazingnow@gmail.com>
Co-Authored-By: kmaclip <kmartclips@proton.me>