[Enhancement] Add regression coverage for ShutdownHookThread and ServerUtil

[lsq3497]

Before Creating the Enhancement Request

• I have confirmed that this should be classified as an enhancement rather than a bug/feature.

Summary

org.apache.rocketmq.srvutil.ShutdownHookThread and ServerUtil are shared utility classes used by RocketMQ entry modules. This change adds focused regression coverage for their observable behavior.

Motivation

These utilities are low-level and easy to regress:

• ShutdownHookThread controls shutdown callback execution
• ServerUtil builds CLI options and converts CommandLine to Properties
• both are reused by multiple entry points

Without regression tests, small changes here may introduce:

• duplicate shutdown callback execution
• unclear behavior when the callback throws
• incorrect command-line option construction
• incorrect conversion from CommandLine to Properties

Describe the Solution You'd Like

Add focused unit tests under srvutil/src/test/java/org/apache/rocketmq/srvutil/ to cover:

• repeated run() invocation semantics in ShutdownHookThread
• callback exception handling in ShutdownHookThread
• ServerUtil.buildCommandlineOptions()
• ServerUtil.commandLine2Properties()

The scope should stay limited to observable behavior. No production logic changes are required.

Describe Alternatives You've Considered

• Testing parseCmdLine() directly
  • Rejected because it can terminate the JVM via System.exit()
  • This makes the test setup less stable and harder to reuse
• Expanding the scope to integration-level startup behavior
  • Rejected because this is a narrow regression task for utility code
  • The lower-cost unit coverage is enough for the intended contribution

Additional Context

Covered behavior:

• repeated run() invocation on ShutdownHookThread
• exception handling in the shutdown callback
• ServerUtil.buildCommandlineOptions()
• ServerUtil.commandLine2Properties()

Not covered:

• parseCmdLine() exit behavior
• JVM shutdown hook registration
• integration-level startup behavior

Acceptance criteria:

• tests added under srvutil/src/test/java/org/apache/rocketmq/srvutil/
• tests pass locally
• no production logic change
• no unrelated refactor

Expected outcome:

• stronger regression protection for srvutil
• safer future changes in shutdown and CLI helper code
• no production behavior change