[Test] Exclude mock initialization from pipe worker timeouts

[Caideyipi]

Description

PipeProcessorSubtaskWorkerTest.testOneHundredWaitingPipesDoNotWaitForWorkerPolling can exhaust its 30-second timeout while a fresh Windows CI fork initializes Mockito/Byte Buddy, before any workers start. The failing CI job stops in Byte Buddy class loading and Mockito plugin initialization through createRunnableSubtask:299.

Initialize a PipeProcessorSubtask mock in @BeforeClass, before JUnit starts either worker-notification test's timer. Both tests keep their existing deadlines, including the 5-second admission wake-up assertion.

Validation

• Clean reactor build and targeted tests: mvn clean test -pl iotdb-core/datanode -am -Dtest=PipeProcessorSubtaskWorkerTest,PipeProcessorSubtaskExecutionGuardTest,PipeMemoryManagerTest -Dsurefire.failIfNoSpecifiedTests=false -DfailIfNoTests=false -Ddevelocity.off=true — 18 tests passed.
• mvn spotless:apply -pl iotdb-core/datanode — passed.
• Injected a 35-second delay into the first Byte Buddy class load in separate JVMs: the original test reproduced the timeout at createRunnableSubtask:299 after 30.0 seconds; the fixed test passed, with 37.3 seconds total including class setup.

This PR has:

• been self-reviewed.
• added comments explaining the intent.
• modified existing tests to address the CI failure.