Fix SharedMem self-deadlock in blocked-processing recovery

[zacheryasc]

Description

get_and_remove_blocked_processing() already holds node_->empty_cv_mutex
when it calls listener_processing_stop(), which attempts to lock the same
non-recursive std::mutex again. This re-entrancy self-deadlocks and spins
inside recover_blocked_processing() (issue #6485).

Clear listeners_status[i].is_processing in place instead, since the mutex is
already held. No mutex type or signature changed, so this is ABI- and
behavior-compatible and safe to backport. Adds a regression test that drives
the recovery path and asserts the listener is cleared without re-blocking.

@Mergifyio backport 3.2.x 2.14.x

Fixes #6485

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• Tests that thoroughly check the new feature have been added/Regression tests checking the bug and its fix have been added; the added tests pass locally
• N/A Any new/modified methods have been properly documented using Doxygen. (no public API change; existing Doxygen block on the method is still accurate)
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• N/A New feature has been added to the versions.md file (if applicable).
• N/A New feature has been documented/Current behavior is correctly described in the documentation.
• Applicable backports have been included in the description.

Reviewer Checklist

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• Check CI results: failing tests are unrelated with the changes.

[IloveZhengYi]

thanks for your work! Hope the maintainers can get this merged soon. 👍