fix flaky test caused by timing issue

[ZejiaJiang]

Description

The eventhub test triggered by core-ci is failed. It's a flaky test.

According to the other tests

azure-sdk-for-java/sdk/servicebus/azure-messaging-servicebus/src/test/java/com/azure/messaging/servicebus/ServiceBusProcessorTest.java

Lines 412 to 414 in cf84f4b

	// This is one less because the processEvent is called before the end span call, so it is possible for
	// to reach this line without calling it the 5th time yet. (Timing issue.)
	verify(tracer, atLeast(numberOfTimes - 1)).end(eq("success"), isNull(), any());

I found that processEvent and endSpan have the timing issue in

azure-sdk-for-java/sdk/eventhubs/azure-messaging-eventhubs/src/main/java/com/azure/messaging/eventhubs/PartitionPumpManager.java

Lines 275 to 284 in cf84f4b

	partitionProcessor.processEvent(new EventContext(partitionContext, eventData, checkpointStore,
	eventContext.getLastEnqueuedEventProperties()));
	if (logger.canLogAtLevel(LogLevel.VERBOSE)) {
	logger.verbose("Completed processing event {}, {}", partitionContext.getEventHubName(),
	partitionContext.getPartitionId());
	}
	endProcessTracingSpan(processSpanContext, Signal.complete());
	} catch (Throwable throwable) {
	/* user code for event processing threw an exception - log and bubble up */
	endProcessTracingSpan(processSpanContext, Signal.error(throwable));

I take the same approach just like the other tests.

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• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

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[ZejiaJiang]

#26759

[ZejiaJiang]

@lmolkova

[haolingdong-msft]

LGTM.

[lmolkova]

@ZejiaJiang thank you!