[ZEPPELIN-6356] Fix ZeppelinClientIntegrationTest

### What is this PR for?

After apache#5090, we started hiding unhandled error details from clients, which caused some tests that depended on those details to fail. The afffected cases are note imports/creations that should fail when the target note path already exists.

This PR introduces a `ConflictException` and updates `NotebookRestApi` to return HTTP 409 (Conflict) when `NotebookService` throws or returns `NotePathAlreadyExistsException`.

I chose to do this exception-to-HTTP mapping in `NotebookRestApi` because `NotebookService` is also used outside REST contexts, and assigning HTTP status codes belongs in the REST layer.


### What type of PR is it?
Bug Fix

### What is the Jira issue?
https://issues.apache.org/jira/browse/ZEPPELIN-6356

### How should this be tested?
- Check if `zeppelin-integration-test` and other tests pass

### Screenshots (if appropriate)

<img width="1609" height="100" alt="image" src="https://github.com/user-attachments/assets/a445f36c-0267-42df-a75e-9f9f05f94964" />


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


Closes apache#5097 from tbonelee/fix-client-integration-test.

Signed-off-by: ChanHo Lee <[email protected]>

Author: tbonelee

zeppelin-server/src/main/java/org/apache/zeppelin/rest/AbstractRestApi.java

@@ -55,6 +55,8 @@ public void onFailure(Exception ex, ServiceContext context) throws IOException {
       super.onFailure(ex, context);
       if (ex instanceof WebApplicationException) {
         throw (WebApplicationException) ex;
+      } else if (ex instanceof IOException) {
+        throw (IOException) ex;
       } else {
         throw new IOException(ex);
       }

zeppelin-server/src/main/java/org/apache/zeppelin/rest/exception/WebApplicationExceptionMapper.java

@@ -22,6 +22,8 @@
 import jakarta.ws.rs.ext.ExceptionMapper;
 import jakarta.ws.rs.ext.Provider;
 
+import org.apache.zeppelin.notebook.exception.NotePathAlreadyExistsException;
+import org.apache.zeppelin.utils.ExceptionUtils;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -34,6 +36,8 @@ public class WebApplicationExceptionMapper implements ExceptionMapper<Throwable>
   public Response toResponse(Throwable exception) {
     if (exception instanceof WebApplicationException) {
       return ((WebApplicationException) exception).getResponse();
+    } else if (exception instanceof NotePathAlreadyExistsException) {
+      return ExceptionUtils.jsonResponseContent(Response.Status.CONFLICT, exception.getMessage());
     } else {
       LOGGER.error("Error response", exception);
       // Return generic error message to prevent information disclosure

zeppelin-server/src/main/java/org/apache/zeppelin/service/NotebookService.java

@@ -385,7 +385,9 @@ public String importNote(String notePath,
           callback.onSuccess(note, context);
           return note.getId();
         });
-
+    } catch (NotePathAlreadyExistsException e) {
+      callback.onFailure(new NotePathAlreadyExistsException("Fail to import note: " + e.getMessage(), e), context);
+      return null;
     } catch (IOException e) {
       callback.onFailure(new IOException("Fail to import note: " + e.getMessage(), e), context);
       return null;

zeppelin-zengine/src/main/java/org/apache/zeppelin/notebook/exception/NotePathAlreadyExistsException.java

@@ -26,4 +26,8 @@ public NotePathAlreadyExistsException(final String message) {
         super(message);
     }
 
+    public NotePathAlreadyExistsException(final String message, final Throwable cause) {
+        super(message, cause);
+    }
+
 }