[ZEPPELIN-6385] Refactor StringsCompleter to use method reference for Comparator

[hyunw9]

What is this PR for?

This PR refactors the anonymous Comparator implementation in StringsCompleter to use a method reference (String::compareToIgnoreCase). The change simplifies the code, improves readability, and aligns the class with modern Java best practices.

What type of PR is it?

Refactoring

Todos

• [x] - Replace anonymous Comparator<String> class with method reference.

What is the Jira issue?

• [ZEPPELIN-6385]

How should this be tested?

Since the expected behavior remains unchanged, no additional tests are required.

Screenshots (if appropriate)

Questions:

• Does the license files need to update? - No
• Is there breaking changes for older versions? - No
• Does this needs documentation? - No