fix(opcua): thread-safety fix for the REST/poll fault-buffer race

[mfaferek93]

Problem

Under UI load the OPC-UA gateway crashed with heap-corruption signatures (GPF /
SIGILL / bad-pointer segfault) inside libros2_medkit_opcua_plugin.so. Root
cause is a data race on pending_reports_, the fire-and-forget fault
report/clear buffer: it is mutated by the poll thread (publish_values,
on_alarm_change/on_event_alarm) AND by the REST thread
(FaultProvider::clear_fault -> send_clear_fault -> send_or_buffer) with no
lock. A push_back that reallocates the vector while the other thread iterates
it is a use-after-free that corrupts the heap.

Fix

Guard every access with a mutex. flush_pending_reports swaps the ready batch
out under the lock and dispatches it outside, so the vector is never reallocated
mid-iteration and the ROS service call never runs under the lock.

Test

OpcuaPluginConcurrency.ClearFaultBufferIsThreadSafe: six threads drive the
REST clear_fault path against the running poll thread. It segfaults on the
unsynchronized code (verified) and passes with the fix. Full unit suite green.

[Copilot]

Pull request overview

This PR addresses a confirmed data race in the OPC-UA plugin’s buffered fault report/clear dispatch queue (pending_reports_) that could trigger heap corruption when the poll thread and REST thread concurrently mutate/iterate the vector.

Changes:

• Add a mutex (pending_reports_mutex_) to serialize all access to pending_reports_.
• Refactor flush_pending_reports() to swap the pending batch under lock and dispatch outside the critical section.
• Add a concurrency regression test that stresses the REST clear_fault path against the running poll thread.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/ros2_medkit_plugins/ros2_medkit_opcua/src/opcua_plugin.cpp	Serializes pending dispatch buffering and flush via a mutex; flush now swaps-and-dispatches outside the lock.
src/ros2_medkit_plugins/ros2_medkit_opcua/include/ros2_medkit_opcua/opcua_plugin.hpp	Documents the two-thread access pattern and introduces pending_reports_mutex_ to protect the buffer.
src/ros2_medkit_plugins/ros2_medkit_opcua/test/test_opcua_plugin.cpp	Adds a multi-threaded regression test to reproduce/guard the REST-vs-poll buffer race.