Plumb `trailingSlash` from the resolved next.config into NextURL via
NextRequest, so `NextResponse.redirect(request.nextUrl)` (and any other
stringification of a NextURL) emits a Location header that matches the
user's slash policy. Mirrors Next.js's `formatNextPathnameInfo`.
Pre-route trailing-slash normalization (e.g. `/foo` -> `/foo/` when
trailingSlash:true) already runs at every entry point via
`normalizeTrailingSlash` in request-pipeline. This commit closes the
remaining gap by making redirect Locations produced from inside
middleware (the common one-hop `request.nextUrl` redirect pattern in
the Next.js middleware-trailing-slash fixture) carry the slash too.
Refs #1332. Sub-problem 3 (shallow navigation triggering a server
roundtrip) is a separate Pages Router client-side concern and deferred
as follow-up.
James Anderson