fix(integration): fix login race condition in Selenium integration tests

The Selenium test InterpreterModeActionsIT.testPerUserIsolatedAction
fails intermittently due to two race conditions:

1. sendKeys may not complete before the login button is clicked,
   causing the server to receive a null username. Fixed by clearing
   the field first and verifying the value is non-empty before submit.

2. The login modal overlay remains visible after authentication,
   causing ElementClickInterceptedException. Fixed by explicitly
   waiting for modal invisibility and removing backdrop via JS.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: jongyoul

zeppelin-integration/src/test/java/org/apache/zeppelin/AbstractZeppelinIT.java

@@ -55,13 +55,36 @@ protected void authenticationUser(String userName, String password) {
         By.xpath("//div[contains(@class, 'navbar-collapse')]//li//button[contains(.,'Login')]"),
         MAX_BROWSER_TIMEOUT_SEC).click();
 
-    visibilityWait(By.xpath("//*[@id='userName']"), MAX_BROWSER_TIMEOUT_SEC).sendKeys(userName);
-    visibilityWait(By.xpath("//*[@id='password']"), MAX_BROWSER_TIMEOUT_SEC).sendKeys(password);
+    WebElement userNameField = visibilityWait(By.xpath("//*[@id='userName']"), MAX_BROWSER_TIMEOUT_SEC);
+    userNameField.clear();
+    userNameField.sendKeys(userName);
+
+    WebElement passwordField = visibilityWait(By.xpath("//*[@id='password']"), MAX_BROWSER_TIMEOUT_SEC);
+    passwordField.clear();
+    passwordField.sendKeys(password);
+
+    // Verify credentials were entered before submitting
+    WebDriverWait fieldWait = new WebDriverWait(manager.getWebDriver(), Duration.ofSeconds(5));
+    fieldWait.until(driver -> !driver.findElement(By.xpath("//*[@id='userName']"))
+        .getAttribute("value").isEmpty());
+
     clickableWait(
         By.xpath("//*[@id='loginModalContent']//button[contains(.,'Login')]"),
         MAX_BROWSER_TIMEOUT_SEC).click();
 
-    ZeppelinITUtils.sleep(1000, false);
+    // Wait for the login modal to fully close before interacting with other elements
+    WebDriverWait wait = new WebDriverWait(manager.getWebDriver(),
+        Duration.ofSeconds(MAX_BROWSER_TIMEOUT_SEC));
+    wait.until(ExpectedConditions.invisibilityOfElementLocated(By.id("loginModal")));
+
+    // Dismiss any lingering modal backdrop via JavaScript
+    try {
+      ((JavascriptExecutor) manager.getWebDriver()).executeScript(
+          "document.querySelectorAll('.modal-backdrop').forEach(e => e.remove());");
+    } catch (Exception e) {
+      // ignore if not ready
+    }
+    ZeppelinITUtils.sleep(500, false);
   }
 
   protected void logoutUser(String userName) throws URISyntaxException {