fix: stop router before NATS in integration test cleanup

This prevents the subscriber's ClosedCB from firing log.Fatal when
NATS is stopped first, which was causing the test process to exit
prematurely and leading to port binding conflicts in parallel test
runs.

The cleanup order is now:
1. Terminate gorouter session
2. Stop NATS server
3. Clean up test files

This matches the fix from upstream PR #555 (commit b2bf830) which
resolved similar issues in router/router_test.go.

Author: rkoster

src/code.cloudfoundry.org/gorouter/integration/common_integration_test.go

@@ -468,7 +468,7 @@ func (s *testState) StartGorouterOrFail() {
 
 func (s *testState) StopAndCleanup() {
 	// Stop router before NATS to prevent subscriber's ClosedCB from
-	// firing log.Fatal → os.Exit(1), which kills the test proc.
+	// firing log.Fatal → os.Exit(1), which kills the test proc
 	if s.gorouterSession != nil && s.gorouterSession.ExitCode() == -1 {
 		Eventually(s.gorouterSession.Terminate(), 5).Should(Exit(0))
 	}