### What is this PR for?
  This PR fixes a critical security vulnerability in JWT token validation where tokens without expiration time were incorrectly accepted as valid, potentially allowing indefinite unauthorized access. The
  fix ensures all JWT tokens must have a valid expiration time and adds comprehensive unit tests to prevent regression.

  ### What type of PR is it?
  Bug Fix

  ### Todos
  * [x] - Fix JWT expiration validation logic to reject null expiration tokens
  * [x] - Add security warning logs for rejected tokens
  * [x] - Create comprehensive unit tests for JWT validation scenarios
  * [x] - Verify backwards compatibility with existing valid tokens

  ### What is the Jira issue?
  * [ZEPPELIN-6266](https://issues.apache.org/jira/browse/ZEPPELIN-6266) Fix JWT expiration validation security vulnerability

  ### How should this be tested?
  * **Automated Unit Tests Added**:
    - New test file: `KnoxJwtRealmTest.java` with 3 comprehensive test scenarios
    - Run: `mvn test -Dtest=KnoxJwtRealmTest -pl zeppelin-server`
    - All tests pass: `Tests run: 3, Failures: 0, Errors: 0, Skipped: 0`

  * **Manual Testing Steps**:
    1. Create JWT token without expiration time → Should be rejected with security warning
    2. Create JWT token with valid future expiration → Should be accepted
    3. Create JWT token with past expiration → Should be rejected
    4. Check server logs for security warnings: "JWT token has no expiration time - rejecting token for security"

  * **Security Validation**:
    - Verify that tokens without expiration are properly rejected
    - Confirm existing valid tokens continue to work
    - Check security event logging

  ### Screenshots (if appropriate)
  N/A - Security fix with no UI changes

  ### Questions:
  * **Does the license files need to update?** No - only modified existing files and added test files with standard Apache license headers
  * **Is there breaking changes for older versions?** No - fully backwards compatible. Only affects tokens with missing expiration (which should not exist in proper JWT implementations)
  * **Does this needs documentation?** No - internal security fix that doesn't change public APIs or configuration

Closes #5007 from chadongmin/ZEPPELIN-6266.

Signed-off-by: Jongyoul Lee <[email protected]>
(cherry picked from commit 4bf5ab9)
Signed-off-by: Jongyoul Lee <[email protected]>