CASSANDRA-21135 Fix JMXFeatureTest failure due to disabled AsyncProfiler

[arvindKandpal-ksolves]

Jira Link: CASSANDRA-21135

Description:
This PR fixes a regression in JMXFeatureTest caused by the recently introduced Async Profiler (CASSANDRA-20854).

Root Cause:
The AsyncProfilerService.status() method was throwing an IllegalStateException when the profiler was disabled (default state). During the JMXFeatureTest, the node restart process triggers JMX discovery/validation. When the test probe accessed the AsyncProfilerMBean status, the exception caused the test to fail.

Fix:

• Modified AsyncProfilerService.status() to return a descriptive String message ("Async Profiler is not enabled...") instead of throwing an exception when disabled.
• This adheres to better JMX practices where attribute getters should be side-effect free and not throw exceptions for valid states (like being disabled).
• Updated AsyncProfilerServiceTest to assert the message string instead of expecting an exception.

Verification:

• Verified locally that org.apache.cassandra.distributed.test.jmx.JMXFeatureTest passes.
• Verified that org.apache.cassandra.service.AsyncProfilerServiceTest passes.

patch by Arvind Kandpal; for CASSANDRA-21135

[smiklosovic]

cc @bbotella