Fix duplicate RUNNING_VM helper record creation on repeated VM.START events

[SURYAS1306]

Description

Repeated VM.START events for the same VM could result in duplicate
RUNNING_VM helper usage records being created in the usage database.

This PR adds a defensive check in UsageManagerImpl#createVMHelperEvent
to detect an existing active RUNNING_VM usage record (same VM instance,
endDate = NULL) and skip helper record creation when a duplicate START
event is received.

A regression test has been added to ensure that duplicate RUNNING_VM
helper records are not created when repeated VM.START events are
processed.

Fixes: #12590

Expected behavior

Only one active RUNNING_VM usage helper record should exist per VM at
any given time.

Actual behavior

Multiple RUNNING_VM helper records could be created for the same VM
when duplicate VM.START events were processed.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• Test (unit or integration test code)

Bug Severity

• Minor

How Has This Been Tested?

• Added a unit test in UsageManagerImplTest to verify that duplicate
  RUNNING_VM helper records are not created when repeated VM.START
  events are handled.
• Built and tested the cloud-usage module successfully using Maven.

How did you try to break this feature and the system with this change?

• Simulated repeated VM.START events for the same VM instance to ensure
  that helper record creation is skipped when an active RUNNING_VM
  record already exists.

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