[MINOR] Refresh search indexes after paragraph edits

[kkalyan]

What is this PR for?

As of now, search index (lucene and embedding) are not updated on paragraph edits.
This change fires fireParagraphUpdateEvent after each successful save, allowing both Lucene and embedding search implementations to refresh the affected paragraph immediately.
Paragraph edits made through NotebookService.updateParagraph or the paragraph REST PUT endpoint are saved but do not fire a paragraph update event. As a result, SearchService implementations keep the old paragraph text until a full index rebuild or server restart.

What type of PR is it?

Bug Fix

Todos

• Refresh search indexes after service-layer paragraph edits
• Refresh search indexes after REST paragraph edits
• Add service coverage with a real Lucene search listener
• Add REST-to-search integration coverage

What is the Jira issue?

No Jira issue has been created yet. This is currently submitted as a [MINOR] bug fix.

How should this be tested?

Automated tests:

./mvnw -pl zeppelin-server -Dtest=NotebookServiceTest,LuceneSearchTest test
./mvnw -pl zeppelin-server -Dtest=ZeppelinRestApiTest#testUpdateParagraph test

The tests update paragraph text through both supported paths and verify that the new text is returned by Lucene search.

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; this restores expected search-index consistency.