fix(next): support HEAD requests in REST routes

[raunak3208]

What?

Adds explicit HEAD support to the Next.js REST route handlers by exporting:

export const HEAD = GET

from packages/next/src/routes/rest/index.ts.

Why?

/api/media/file/* routes were returning 404 for HEAD requests while returning 200 for the same resource with GET.

This causes inconsistent behavior for clients and proxies using HEAD requests for lightweight existence checks and does not align with RFC 9110 §9.3.2 behavior expectations.

How?

The REST route handlers already export GET, POST, PATCH, etc., but did not explicitly export a HEAD handler.

This PR maps HEAD to the existing GET handler so that HEAD requests follow the same routing and response logic without returning a response body.

Fixes #16754