[ZEPPELIN-6453] Handle secondary NotebookRepo remove failures consistently

### What is this PR for?
In `NotebookRepoSync`, secondary storage failures were already handled for `save()` and `move()`, but `remove()` still used a generic `for` loop with a `TODO` comment for handling secondary remove failures.

This PR makes the `remove()` failure handling consistent with the existing `save()` and `move()` behavior:
* **Primary Storage (`getRepo(0)`)**: A failure during removal stops the operation and propagates the `IOException`.
* **Secondary Storage (`getRepo(1)`)**: A failure during removal is caught and explicitly logged, preventing silent inconsistencies without interrupting the overall operation.

To achieve this, the `for` loop in `remove(String noteId, String notePath, AuthenticationInfo subject)` was replaced with explicit `getRepo(0)` and `getRepo(1)` calls, and the outstanding `TODO` was removed.

### What type of PR is it?
Improvement

### What is the Jira issue?
* https://issues.apache.org/jira/browse/ZEPPELIN-6453

### How should this be tested?
Tested via `NotebookRepoSyncTest` using Mockito to explicitly verify the primary vs. secondary failure behaviors:
* `testRemoveSucceedsWhenSecondaryRepoFails`: Verifies that an `IOException` from the secondary repo is caught and does not stop the removal process.
* `testRemoveFailsWhenPrimaryRepoFails`: Verifies that an `IOException` from the primary repo halts the operation and propagates the exception.
* All existing tests in `NotebookRepoSyncTest` continue to pass.

### Screenshots (if appropriate)
N/A

### Questions:
* Does the license files need to update? No
* Is there breaking changes for older versions? No
* Does this needs documentation? No

Closes #5289 from move-hoon/ZEPPELIN-6453-remove-secondary-failure.

Signed-off-by: Jongyoul Lee <jongyoul@gmail.com>

Author: move-hoon

zeppelin-server/src/main/java/org/apache/zeppelin/notebook/repo/NotebookRepoSync.java

@@ -208,10 +208,15 @@ public void move(String folderPath, String newFolderPath,
 
   @Override
   public void remove(String noteId, String notePath, AuthenticationInfo subject) throws IOException {
-    for (NotebookRepo repo : repos) {
-      repo.remove(noteId, notePath, subject);
+    getRepo(0).remove(noteId, notePath, subject);
+    if (getRepoCount() > 1) {
+      try {
+        getRepo(1).remove(noteId, notePath, subject);
+      }
+      catch (IOException e) {
+        LOGGER.info("{}: Failed to remove from secondary storage", e.getMessage());
+      }
     }
-    /* TODO(khalid): handle case when removing from secondary storage fails */
   }
 
   @Override

zeppelin-server/src/test/java/org/apache/zeppelin/notebook/repo/NotebookRepoSyncTest.java

@@ -18,12 +18,20 @@
 package org.apache.zeppelin.notebook.repo;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.doThrow;
 import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.never;
+import static org.mockito.Mockito.times;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
 
 import java.io.File;
 import java.io.IOException;
 import java.nio.file.Files;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Set;
@@ -434,4 +442,57 @@ void testSyncWithAcl() throws IOException {
     assertEquals(0, authorizationService.getRunners(noteId).size());
     assertEquals(0, authorizationService.getWriters(noteId).size());
   }
+
+  @Test
+  void testRemoveSucceedsWhenSecondaryRepoFails() throws IOException {
+    NotebookRepo primaryRepo = mock(NotebookRepo.class);
+    NotebookRepo secondaryRepo = mock(NotebookRepo.class);
+    doThrow(new IOException("secondary remove failed"))
+        .when(secondaryRepo).remove("noteId", "notePath", anonymous);
+
+    try (NotebookRepoSync repoSync = newSyncWithRepos(primaryRepo, secondaryRepo)) {
+      // secondary storage failure must not fail the whole operation
+      repoSync.remove("noteId", "notePath", anonymous);
+
+      verify(primaryRepo, times(1)).remove("noteId", "notePath", anonymous);
+      verify(secondaryRepo, times(1)).remove("noteId", "notePath", anonymous);
+    }
+  }
+
+  @Test
+  void testRemoveFailsWhenPrimaryRepoFails() throws IOException {
+    NotebookRepo primaryRepo = mock(NotebookRepo.class);
+    NotebookRepo secondaryRepo = mock(NotebookRepo.class);
+    doThrow(new IOException("primary remove failed"))
+        .when(primaryRepo).remove("noteId", "notePath", anonymous);
+
+    try (NotebookRepoSync repoSync = newSyncWithRepos(primaryRepo, secondaryRepo)) {
+      // primary storage failure must stop the operation and propagate the exception
+      assertThrows(IOException.class,
+          () -> repoSync.remove("noteId", "notePath", anonymous));
+
+      verify(primaryRepo, times(1)).remove("noteId", "notePath", anonymous);
+      verify(secondaryRepo, never()).remove("noteId", "notePath", anonymous);
+    }
+  }
+
+  /**
+   * Builds a NotebookRepoSync backed by the two given repos, injected through a mocked
+   * PluginManager so the real init() path is exercised without touching internal fields.
+   */
+  private NotebookRepoSync newSyncWithRepos(NotebookRepo primaryRepo, NotebookRepo secondaryRepo)
+      throws IOException {
+    PluginManager mockPluginManager = mock(PluginManager.class);
+    when(mockPluginManager.loadNotebookRepo("primaryRepo")).thenReturn(primaryRepo);
+    when(mockPluginManager.loadNotebookRepo("secondaryRepo")).thenReturn(secondaryRepo);
+    // list() is queried during the init-time anonymous sync; keep it a no-op
+    when(primaryRepo.list(any())).thenReturn(new HashMap<>());
+    when(secondaryRepo.list(any())).thenReturn(new HashMap<>());
+
+    zConf.setProperty(ConfVars.ZEPPELIN_NOTEBOOK_STORAGE.getVarName(),
+        "primaryRepo,secondaryRepo");
+    NotebookRepoSync repoSync = new NotebookRepoSync(mockPluginManager);
+    repoSync.init(zConf, noteParser);
+    return repoSync;
+  }
 }