fix(vercel): run edge middleware when isr is enabled

[asmyshlyaev177]

Changes

With isr enabled, middlewareMode: 'edge' is silently inert.

The adapter builds and deploys _middleware.func, but every on-demand route's
dest is set to the ISR function, so nothing ever routes to it. Middleware
still runs — but only inside the ISR function, which Vercel skips entirely on a
cache hit. The observable result is middleware that works on a cold entry and
then stops running once the entry is warm, with no error and no log.

This is the ordering problem: middleware has to run before the cache is
consulted, not behind it.

• Route on-demand pages at _middleware rather than _isr when a middleware
  entry point exists, so the edge function is actually reached.
• Collect those route patterns and inline them into the generated middleware,
  so next() forwards to /_isr?x_astro_path=… for a route the ISR function
  backs, and /_render for one it doesn't. Cached responses are still served
  from cache; only the entry point moves.
• _image and _server-islands keep going straight to _render, unchanged.
• Routes matched by isr.exclude still resolve to _render through next().
• Prerendered pages are untouched: no route entry, served as static files.

x_astro_path carries the original pathname, so the ISR cache key stays the
request path rather than collapsing to /_isr; x_astro_path_token is the
build token added in #17370. Without isr, or without a middleware file,
nothing about the output changes.

Testing

New: packages/integrations/vercel/test/isr-edge-middleware.test.ts — 16 tests
over two fixtures.

isr-with-edge-middleware asserts against the real build output, since the bug
is entirely a property of the emitted config.json:

• pages, dynamic routes, endpoints and the 404 resolve to _middleware
• _image and _server-islands still resolve to _render
• prerendered pages get no route entry and ship as static HTML
• configured redirects still resolve ahead of the middleware
• the ISR prerender config and its expiration survive

and then imports the generated middleware.mjs with fetch stubbed, to check
where next() actually forwards:

• a cached route → /_isr, with x_astro_path and a token
• a dynamic route → /_isr?x_astro_path=/cached/42, the real path, because that
  path is the cache key
• an isr.exclude route → /_render
• a query string does not leak into x_astro_path
• the response still reaches the middleware, headers intact

isr-edge-no-middleware covers middlewareMode: 'edge' with no middleware file
present: routes go straight to _isr as before and no middleware function is
built.

Verified as a regression guard by reverting src/ and re-running: 4 of the 16
fail, all of them on dest.

Also ran the full suite — core unit (3307), core integration (1240), all 18
integration packages including @astrojs/vercel (58), language-tools (98), and
e2e in chrome and firefox. No new failures; the pre-existing ones
(test/fonts.test.ts cancellations, a handful of e2e) don't touch the adapter.

One caveat worth stating plainly: this is verified against build output and the
generated module, not against a live Vercel deployment. Confirmation on a real
project would be welcome.

[changeset-bot]

🦋 Changeset detected

Latest commit: d9b0c39

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 23 packages

Name	Type
@astrojs/vercel	Patch
@test/astro-vercel-basic	Patch
@test/astro-vercel-image	Patch
@test/astro-vercel-integration-assets	Patch
@test/vercel-isr-edge-no-middleware	Patch
@test/vercel-isr-with-edge-middleware	Patch
@test/vercel-isr	Patch
@test/vercel-max-duration	Patch
@test/vercel-edge-middleware-with-edge-file	Patch
@test/vercel-edge-middleware-without-edge-file	Patch
@test/astro-vercel-no-output	Patch
@test/astro-vercel-prerendered-error-pages	Patch
@test/astro-vercel-redirects-serverless	Patch
@test/astro-vercel-redirects	Patch
@test/vercel-server-islands	Patch
@test/astro-vercel-serverless-prerender	Patch
@test/astro-vercel-serverless-with-dynamic-routes	Patch
@test/astro-vercel-static-assets	Patch
@test/vercel-static-headers	Patch
@test/astro-vercel-static	Patch
@test/vercel-streaming	Patch
@test/astro-vercel-with-web-analytics-enabled-output-as-static	Patch
vercel-hosted-astro-project	Patch

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