fix: head requests to /api/media/file/:filename return 404 instead of 200 (RFC 9110 §9.3.2)

[Asadullah-shz]

Problem

Per RFC 9110 §9.3.2, a HEAD response must be identical to GET except for
the absence of a response body. However, HEAD /api/media/file/:filename
returned 404 while GET returned 200 for the exact same URL.

URL='https://your-site.example.com/api/media/file/portrait.jpg'
curl -sI "$URL" | head -n1
# Observed: HTTP/2 404
# Expected: HTTP/2 200
curl -s -o /dev/null -w '%{http_code}\n' "$URL"
# Observed: 200 (GET works fine)
This causes operator scripts and upstream proxies that use HEAD as a cheap existence probe to report false 404s.

**Root Cause**
Two compounding issues:

HEAD was not exported from @payloadcms/next/routes — so Next.js App Router had no explicit HEAD handler, falling back to its auto-synthesis which failed to reach the media file route.

handleEndpoints passed the full body back for HEAD — the HEAD→GET routing fallback correctly found the getFileHandler, but the handler opened an fs.createReadStream and returned a Response with a full body stream. The body was passed through unchanged, leaking an open file handle and producing a non-RFC-compliant response.

**Fix**
packages/next/src/routes/rest/index.ts
Export HEAD = handlerBuilder so Next.js App Router registers an explicit HEAD handler at the route level.

packages/next/src/routes/index.ts + packages/next/src/exports/routes.ts
Re-export HEAD as REST_HEAD so consumers can import it explicitly in their app/(payload)/api/[...slug]/route.ts.

packages/payload/src/utilities/handleEndpoints.ts
When HEAD is resolved via the GET fallback:

Cancel the body stream immediately — releases the fs.createReadStream file handle, prevents resource leaks.
Return new Response(null, ...) — body-less, but with identical Content-Type, Content-Length, Accept-Ranges, and status code, fully compliant with RFC 9110 §9.3.2.
test/__helpers/shared/NextRESTClient.ts
Add a HEAD() method wired to REST_HEAD so integration tests can issue HEAD requests end-to-end.

test/uploads/int.spec.ts
Add regression test verifying:

HEAD /media/file/:filename → 200
content-type header matches (image/png)
Uploaded document is cleaned up after the test