fix: set restart monitor before checking for dev server connection (#17157)

If hot reload is enabled, e.g. with Spring Dev Tools, during Java server restart
the check for active dev-server connection may temporarily fail because the
dev-server is restarting.
This change finds and sets the currently active restart monitor before checking
for dev-server connection, so that the check will wait for a potential restart
to complete.

Author: mcollovati

vaadin-dev-server/src/main/java/com/vaadin/base/devserver/AbstractDevServerRunner.java

@@ -176,6 +176,10 @@ private void runOnFutureComplete() {
     }
 
     void doStartDevModeServer() throws ExecutionFailedException {
+        waitForRestart = DevServerOutputTracker.activeServerRestartGuard();
+        if (waitForRestart != null) {
+            getLogger().debug("RestartMonitor is active");
+        }
         // If port is defined, means that the dev server is already running
         if (port > 0) {
             if (!checkConnection()) {
@@ -537,10 +541,6 @@ private void reuseExistingPort(int port) {
         // Save running port for next usage
         saveRunningDevServerPort();
         watchDog.set(null);
-        waitForRestart = DevServerOutputTracker.activeServerRestartGuard();
-        if (waitForRestart != null) {
-            getLogger().debug("RestartMonitor is active");
-        }
     }
 
     private void saveRunningDevServerPort() {

vaadin-dev-server/src/test/java/com/vaadin/base/devserver/RestartMonitorTest.java

@@ -40,7 +40,7 @@ public void waitForServerReady_serverReady_notBlocking()
         CompletableFuture.runAsync(() -> simulateTask(latch));
         Assert.assertTrue(
                 "Not restarting, execution should not have been blocked",
-                latch.await(5, TimeUnit.MILLISECONDS));
+                latch.await(10, TimeUnit.MILLISECONDS));
     }
 
     @Test