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

The Selenium test InterpreterModeActionsIT.testPerUserIsolatedAction
fails intermittently due to multiple race conditions in the login flow:

1. Pre-existing modal backdrop blocks navbar Login button click.
   Fixed by dismissing any modal/backdrop via JS before starting.

2. sendKeys may not populate the field if Angular hasn't finished
   initializing the form. Fixed by falling back to JavaScript-based
   value injection with Angular input event trigger.

3. After login, wait for the user dropdown to appear (proves login
   succeeded and DOM updated) instead of waiting for modal invisibility
   which can race with Angular's digest cycle.

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

Author: jongyoul

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

@@ -51,17 +51,61 @@ abstract public class AbstractZeppelinIT {
   protected static final long MAX_PARAGRAPH_TIMEOUT_SEC = 120;
 
   protected void authenticationUser(String userName, String password) {
+    // Dismiss any pre-existing modal backdrop that may block clicks
+    try {
+      ((JavascriptExecutor) manager.getWebDriver()).executeScript(
+          "document.querySelectorAll('.modal-backdrop').forEach(e => e.remove());"
+          + "$('#loginModal').modal('hide');");
+      ZeppelinITUtils.sleep(500, false);
+    } catch (Exception e) {
+      // ignore if jQuery/Bootstrap not ready
+    }
+
     clickableWait(
         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 — use JS as a fallback
+    ZeppelinITUtils.sleep(300, false);
+    String enteredUser = userNameField.getAttribute("value");
+    if (enteredUser == null || enteredUser.isEmpty()) {
+      ((JavascriptExecutor) manager.getWebDriver()).executeScript(
+          "document.getElementById('userName').value = arguments[0];"
+          + "angular.element(document.getElementById('userName')).triggerHandler('input');",
+          userName);
+      ((JavascriptExecutor) manager.getWebDriver()).executeScript(
+          "document.getElementById('password').value = arguments[0];"
+          + "angular.element(document.getElementById('password')).triggerHandler('input');",
+          password);
+      ZeppelinITUtils.sleep(200, false);
+    }
+
     clickableWait(
         By.xpath("//*[@id='loginModalContent']//button[contains(.,'Login')]"),
         MAX_BROWSER_TIMEOUT_SEC).click();
 
-    ZeppelinITUtils.sleep(1000, false);
+    // Wait for the logged-in navbar user dropdown to appear
+    visibilityWait(
+        By.xpath("//div[contains(@class, 'navbar-collapse')]//li//button[contains(@class, 'nav-btn dropdown-toggle ng-scope')]"),
+        MAX_BROWSER_TIMEOUT_SEC);
+
+    // Dismiss any lingering modal backdrop via JavaScript
+    try {
+      ((JavascriptExecutor) manager.getWebDriver()).executeScript(
+          "document.querySelectorAll('.modal-backdrop').forEach(e => e.remove());"
+          + "$('#loginModal').modal('hide');");
+    } catch (Exception e) {
+      // ignore if jQuery/Bootstrap not ready
+    }
+    ZeppelinITUtils.sleep(500, false);
   }
 
   protected void logoutUser(String userName) throws URISyntaxException {