fix(cli): add dial timeout and keepalive for Coder Connect (#24015)

The default `net.Dialer` in the Coder Connect path had no timeout,
falling back to the OS TCP timeout when the tunnel was broken but DNS
still resolved. Add a 5s dial timeout and 30s TCP keepalive.

Fixes #24006

cli/ssh.go

@@ -1647,7 +1647,11 @@ func WithTestOnlyCoderConnectDialer(ctx context.Context, dialer coderConnectDial
 func testOrDefaultDialer(ctx context.Context) coderConnectDialer {
 	dialer, ok := ctx.Value(coderConnectDialerContextKey{}).(coderConnectDialer)
 	if !ok || dialer == nil {
-		return &net.Dialer{}
+		// Timeout prevents hanging on broken tunnels (OS default is very long).
+		return &net.Dialer{
+			Timeout:   5 * time.Second,
+			KeepAlive: 30 * time.Second,
+		}
 	}
 	return dialer
 }