MINIFICPP-2791 Fix deadlock during C2 flow update

[lordgamez]

https://issues.apache.org/jira/browse/MINIFICPP-2791

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[Copilot]

Pull request overview

Fixes a reported deadlock during C2-driven flow updates by changing the shutdown/reload ordering around controller services and by making the ProcessGroup on-schedule retry timer teardown thread-safe.

Changes:

• Parse new flow payload before stopping the running flow, then clear controller services only after timer/scheduler threads are stopped (FlowController).
• Add a dedicated mutex for ProcessGroup::onScheduleTimer_ access and restructure timer stop/destroy logic to avoid stop/join deadlocks (ProcessGroup).
• Adjust CallBackTimer destructor locking to avoid potential destructor-time lock contention.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
libminifi/src/FlowController.cpp	Reorders flow update steps to defer controller-service clearing until after schedulers/timers are stopped.
libminifi/src/core/ProcessGroup.cpp	Adds synchronization and a safer stop/destroy loop for the onSchedule retry timer to prevent deadlocks.
libminifi/include/core/ProcessGroup.h	Introduces on_schedule_timer_mutex_ to guard onScheduleTimer_.
core-framework/src/utils/CallBackTimer.cpp	Simplifies destructor by removing an extra mutex lock before joining the thread.

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