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/workspaceapps/proxy.go

@@ -438,7 +438,7 @@ func (s *Server) HandleSubdomain(middlewares ...func(http.Handler) http.Handler)
 			}
 
 			// Step 2: Get the request Host.
-			host := httpapi.RequestHost(r)
+			host := httpmw.EffectiveHost(s.RealIPConfig, r)
 			if host == "" {
 				if r.URL.Path == "/derp" {
 					// The /derp endpoint is used by wireguard clients to tunnel