[ZEPPELIN-5994] Fix cron tests for ZeppelinRestApiTest

[namuuCY]

What is this PR for?

This PR fixes and re-enables the cron REST API tests that were disabled in ZeppelinRestApiTest.

Cron scheduling is disabled for security when Zeppelin runs in anonymous mode, even if the cron configuration is enabled. This PR separates the cron tests by authentication mode:

• Verify that cron creation and deletion are forbidden in anonymous mode.
• Add AuthenticatedCronRestApiTest, which starts Zeppelin with Shiro authentication enabled.
• Verify the cron lifecycle for an authenticated administrator.
• Verify cron folder restrictions and invalid cron expression handling.

What type of PR is it?

Bug Fix

Todos

• Verify that cron operations are rejected in anonymous mode.
• Add authenticated cron REST API tests.
• Test cron folder restrictions.
• Test valid and invalid cron expressions.

What is the Jira issue?

https://issues.apache.org/jira/browse/ZEPPELIN-5994

How should this be tested?

Run the focused REST API tests:

./mvnw -pl zeppelin-server \
  -Dtest=AuthenticatedCronRestApiTest \
  test

./mvnw -pl zeppelin-server \
  -Dtest='ZeppelinRestApiTest#testCronDisabledInAnonymousMode' \
  test

Questions:

• Does the license files need to update? No.
• Is there breaking changes for older versions? No.
• Does this needs documentation? No. This PR only updates test coverage.