[BUG] Demo installer tests are broken

[DarshitChanpura]

Description

With recent update to JSM, the demo config installer tests saw failures with message like:

SecuritySettingsConfigurerTests > testUpdateAdminPasswordWithShortPassword FAILED
    java.lang.UnsupportedOperationException: The Security Manager is deprecated and will be removed in a future release
        at __randomizedtesting.SeedInfo.seed([C764CEC2F75704B:8AE71962A5892A2D]:0)
        at java.base/java.lang.System.setSecurityManager(System.java:430)
        at org.opensearch.security.tools.democonfig.SecuritySettingsConfigurerTests.testUpdateAdminPasswordWithShortPassword(SecuritySettingsConfigurerTests.java:158)

Ref CI run: https://github.com/opensearch-project/security/actions/runs/14367419774/job/40283565423?pr=5257

Proposed Solution

Replace the System exit handler with a custom Exit Handler.