Share the ADMIN text and KILL reason, and test the review's gaps

ADMIN's location and email lines come from service.AdminLines, and
KillUser applies the default reason, so both transports send the same
reply. The Disconnect test now allows a quarter of one write window,
so a Disconnect that writes before returning fails it. QueryUserMode
and UserhostReply are tested with a read that fails. The handler tests
again cover USERHOST for a nick with no session, MODE +o on your own
nick, and the status of a successful KILL. handleMode's comment says
what it does.

Model: opus-5-5

internal/service/service_test.go

@@ -406,6 +406,16 @@ func TestQueryUserMode(t *testing.T) {
 	if modes != "+ow" {
 		t.Errorf("expected +ow, got %s", modes)
 	}
+
+	// A flag that cannot be read is an error, not a flag
+	// that is unset.
+	cancelled, cancel := context.WithCancel(ctx)
+	cancel()
+
+	_, err = env.svc.QueryUserMode(cancelled, sid)
+	if !errors.Is(err, context.Canceled) {
+		t.Errorf("expected the read's error, got %v", err)
+	}
 }
 
 func TestUserhostReply(t *testing.T) {
@@ -453,6 +463,16 @@ func TestUserhostReply(t *testing.T) {
 	if strings.Count(reply, "bob=") != 5 {
 		t.Errorf("want 5 entries for 6 nicks, got %q", reply)
 	}
+
+	// A nick that cannot be looked up is an error, not a nick
+	// with no session.
+	cancelled, cancel := context.WithCancel(ctx)
+	cancel()
+
+	_, err = env.svc.UserhostReply(cancelled, []string{"alice"}, "srv")
+	if !errors.Is(err, context.Canceled) {
+		t.Errorf("want the lookup's error, got %v", err)
+	}
 }
 
 // userModes is the stored state of a session's user mode