fix!: only trust x-forwarded-host from configured trusted proxies (#26204)

Subdomain app routing derived the app identity from
httpapi.RequestHost, which returned the client-supplied
X-Forwarded-Host header verbatim. No middleware validated or stripped
that header, so a request from an untrusted peer could forge it. Since
the application_connect cookie is scoped to the wildcard apps domain,
JavaScript in a share=authenticated app could fetch() with a forged
X-Forwarded-Host pointing at a victim's owner-only app; coderd routed
and authorized the request as the victim and returned the private app
response same-origin to the attacker.

Replace RequestHost with httpmw.EffectiveHost, which honors
X-Forwarded-Host only when the original socket peer is a configured
trusted origin, otherwise falling back to the received Host header.
This ties host trust to the same RealIPConfig model already used for
X-Forwarded-For and -Proto. Wire it into HandleSubdomain for both
coderd and wsproxy, and log both the effective host and the raw
received_host.

Add coverage: EffectiveHost unit tests assert the trust decision uses
the socket peer rather than the spoofable forwarded client IP, and a
HandleSubdomain test confirms a forged X-Forwarded-Host from an
untrusted peer never reaches token resolution.

Refs: https://linear.app/codercom/issue/PLAT-259

coderd/httpapi/request.go

@@ -8,17 +8,6 @@ const (
 	XForwardedHostHeader = "X-Forwarded-Host"
 )
 
-// RequestHost returns the name of the host from the request.  It prioritizes
-// 'X-Forwarded-Host' over r.Host since most requests are being proxied.
-func RequestHost(r *http.Request) string {
-	host := r.Header.Get(XForwardedHostHeader)
-	if host != "" {
-		return host
-	}
-
-	return r.Host
-}
-
 func IsWebsocketUpgrade(r *http.Request) bool {
 	vs := r.Header.Values("Upgrade")
 	for _, v := range vs {