fix(fcp): Avoid input-output lock-order deadlock

[leumor]

Summary

• break the FCP output-thread lock inversion by reading handler.isClosed() before acquiring outQueue in nextQueueAction()
• add a regression test that asserts isClosed() is never invoked while holding outQueue
• preserve existing queue behavior while removing the outQueue -> handler ordering that deadlocked against input-side handler -> outQueue paths

How To Test

• ./gradlew test --tests *FCPConnectionOutputHandlerTest
• ./gradlew test

Context

• investigated threads_report.txt and found repeated mutual wait cycles between FCP input/output handlers (ArrayDeque vs FCPConnectionHandler monitor)
• this patch targets that lock-order cycle directly

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud