fix: detect proxy.ts correctly with compound pageExtensions - #93246

Verifies that proxy.page.ts is correctly detected and executed when
pageExtensions includes compound extensions like ["page.ts"].

Ref vercel#85648
Ref vercel#86303

Apply the same fix as the Turbopack/Rust side: use split('.')[0] instead
of path.parse().name to extract the convention file basename. Handles
compound pageExtensions like 'page.ts' where proxy.page.ts yields
path.parse().name === 'proxy.page' instead of 'proxy'.

Fixes both build-time detection (build/index.ts) and dev-time detection
(setup-dev-bundler.ts).

Fixes vercel#86303
Ref vercel#86122

The original PR description claimed the same fileBaseName extraction is used
for `middleware` and `instrumentation` convention file detection, but the
test fixture only exercised `proxy.page.ts`. Add coverage for the other two
to lock in the fix end-to-end.

  * Add `instrumentation.page.ts` to the existing `proxy-page-extensions`
    fixture and assert its `register()` runs (visible via cliOutput).
  * Add a separate `middleware-page-extensions` fixture so middleware can
    be detected without colliding with proxy (the build refuses both).

Verified locally in dev/turbopack, dev/webpack, start/turbopack, and
start/webpack — 5/5 tests pass in each mode.

…s, tighten existing assertions

New fixtures:
- `proxy-mixed-page-extensions/`: pageExtensions = ['page.tsx', 'tsx']
  (compound + simple coexisting). Locks in that the compound rule
  recognizes `proxy.page.tsx` when a simple entry is also present, and
  that simple-extension routes (`app/page.tsx`) keep working alongside it.
- `proxy-and-middleware-conflict/`: both `proxy.page.ts` and
  `middleware.page.ts` present. Asserts the dual-file error
  ("are detected. Please use \"./proxy.page.ts\" only.") fires in both
  dev and start modes — previously the throw sites at build/index.ts:1285
  and setup-dev-bundler.ts:491 were unreachable on canary and so untested.

Existing test improvements:
- `middleware-page-extensions.test.ts`: assert the deprecation warnOnce
  ("middleware" file convention is deprecated) appears in cliOutput.
  Same rationale — previously unreachable on canary.
- `proxy-page-extensions.test.ts`:
  - Set `x-proxy-ran` response header in `proxy.page.ts` and assert it
    in the "render the page through the proxy" test. Previously the
    test asserted only the page body, which would render even with a
    broken proxy (the test name overstated coverage).
  - Drop the misleading `await next.fetch('/')` and accompanying comment
    in the instrumentation test — `register()` runs at server boot,
    not on first request, so the fetch is decorative.

…nsions-detection

# Conflicts:
#	crates/next-api/src/middleware.rs