fix: wait for dev server in DevModeNoClassCacheIT to prevent flaky test (#22689)

The test was timing out waiting for the "last-span" element because it
didn't wait for the Vite dev server to fully initialize before checking
for DOM elements. Adding waitForDevServer() ensures the dev server,
Spring Boot app, and view rendering complete before assertions.

Author: Artur-

flow-tests/test-redeployment-no-cache/src/test/java/com/vaadin/flow/uitest/ui/DevModeNoClassCacheIT.java

@@ -36,6 +36,7 @@ protected String getTestPath() {
     @Test
     public void testDevModeClassCacheNotPopulated() {
         open();
+        waitForDevServer();
 
         waitForElementPresent(By.id("last-span"));