[ZEPPELIN-6340] Add ZeppelinEventBus and update NotebookServer to handle NoteRemoveEvent

[seung-00]

What is this PR for?

This PR is the first step toward migrating Zeppelin’s listener-based event handling to an EventBus model, as outlined in the proposal. It sets up the EventBus infrastructure and migrates NoteRemoveEvent handling to it.

Key changes:

• Added the ZeppelinEventBus class
• Added RxJava 3 as a dependency of the zeppelin-zengine module
• Added the zeppelin.eventbus.enabled feature flag
• Updated NotebookServer to handle NoteRemoveEvent through ZeppelinEventBus when the feature flag is enabled

This PR supersedes #5085. The original branch had fallen behind the current codebase, and the implementation has since changed, so I opened a new PR.

What type of PR is it?

Improvement

Todos

• Refactor other NoteEventListener methods and additional listeners to EventBus

What is the Jira issue?

• Open an issue on Jira https://issues.apache.org/jira/browse/ZEPPELIN-6340

How should this be tested?

• Run the existing test suite against the updated code
• Verify the note deletion flow through the UI

Screenshots (if appropriate)

Questions:

• Does the license files need to update? N
• Is there breaking changes for older versions? N
• Does this needs documentation? N

[tbonelee]

A few thoughts on subscription registration, since 6341, 6346 and 6348 will likely follow this PR as their model.

observe(NoteEvent.class) receives every subtype of NoteEvent. Once 6348 adds NoteCreatedEvent, this subscription gets it too and falls through to the else branch in handleNoteEvent, logging "Unknown event type" for what is normal behavior. Each new event also means another ticket editing the same if-else chain.

The try/catch in the lambda is load-bearing in a way that is easy to miss. LambdaObserver disposes the subscription when the consumer throws and forwards the error to RxJavaPlugins.onError, which by default only reaches stderr, so a dead subscription leaves no trace in the logs.

Would something like subscribe(Class<T>, Consumer<T>) on EventBus work here? Subscribing by concrete type lets ofType() do the filtering instead of instanceof, and the try/catch sits in the bus once however many subscriptions there are. One caveat: Consumer would need to be io.reactivex.rxjava3.functions.Consumer, since java.util.function.Consumer cannot throw checked exceptions and handlers would go on catching IOException themselves.