RTEMS | rtems_message_queue_receive(RTEMS_NO_WAIT) in ISR corrupts unrelated task Event wait (#5792)

[Sebastian Huber (@sebhub)]

Issue created by Sebastian Huber: https://gitlab.rtems.org/rtems/rtos/rtems/-/work_items/5792



In an interrupt, the immediate path of `_CORE_message_queue_Seize()` stores the priority of the message in `Wait.count` of the interrupted thread. If that thread is in the intend to block window of `_Event_Seize()`, the store replaces its event condition. The event wait of that thread then ends only with a timeout.

`_POSIX_Message_queue_Receive_support()` reads the priority from `Wait.count` after it releases the queue lock. An interrupt which receives a message in between changes the priority which `mq_receive()` returns.

Backport of #5789 to the 6.3 branch. This description was created with Claude Code assistance.

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View it on GitLab: https://gitlab.rtems.org/rtems/rtos/rtems/-/work_items/5792
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