server: assert timeouts on the served http.Server, not just the constructor (closes #120)

The timeout tests called newHTTPServer directly, so a Run that built
its http.Server inline would drop every timeout with the suite still
green. TestRunWiresSocketTimeouts wires a Server as cmd/dnswatcher
does, minus the watcher and resolver so no live DNS is touched, drives
Run with an unbindable port so it stores its http.Server and returns
without listening, and checks that server carries all four timeouts
and both required relationships. The addr/handler test, which could
not fail, is dropped.

The ReadTimeout note now says what net/http does: a request whose
headers arrive after ReadTimeout but within ReadHeaderTimeout gets a
read deadline that has already passed, so reading its body fails at
once.

Model: opus-4-8 (implementation); opus-5-5 (rework)

internal/server/export_test.go

@@ -5,15 +5,20 @@ import (
 	"time"
 )
 
-// NewHTTPServer exports newHTTPServer for testing.
-func NewHTTPServer(
-	listenAddr string,
-	handler http.Handler,
-) *http.Server {
-	return newHTTPServer(listenAddr, handler)
-}
-
 // RequestTimeout exports the handler execution budget applied by
 // chimw.Timeout in SetupRoutes, so tests can assert the relationship
 // between it and the server's WriteTimeout.
 const RequestTimeout time.Duration = requestTimeout
+
+// SetListenPort overrides the port Run binds. A test uses it to hand
+// Run an unbindable port so ListenAndServe fails immediately and Run
+// returns after storing its http.Server.
+func SetListenPort(s *Server, port int) {
+	s.port = port
+}
+
+// HTTPServerOf returns the http.Server that Run built and stored, so a
+// test can inspect the timeouts the running server actually carries.
+func HTTPServerOf(s *Server) *http.Server {
+	return s.httpServer
+}