NIFI-16378: Handle disabled Processors during Connector drain

[markap14]

Summary

• Connector drain never finished when the Connector flow contained a disabled Processor, because Processor lifecycle stop() always waited on the scheduler.
• StandaloneProcessorLifecycle.stop() now returns a completed future when the Processor is already physically stopped or disabled, matching Process Group stop handling.
• ConnectorDrainIT.testDrainFlowFiles now includes a disabled Processor in the gated drain flow so this path is covered.

Test plan

• ConnectorDrainIT#testDrainFlowFiles failed with a timeout while waiting for drain to complete on the original code
• Same test passes after the lifecycle change
• ConnectorDrainIT (full class) passes
• nifi-framework-core unit tests, Checkstyle, and PMD pass

Jira: https://issues.apache.org/jira/browse/NIFI-16378