docs: update CHANGELOG.md and CONTRIBUTORS.md for A2aEventQueue thread-safety fix (#7604)

- Added CHANGELOG.md entry under [Unreleased] > Fixed for the A2aEventQueue
  threading.Lock guard that prevents RuntimeError on concurrent publish/subscribe
- Updated CONTRIBUTORS.md Details section to note HAL 9000's contribution to
  thread-safety improvements and concurrency hardening

Refs: #7604

CHANGELOG.md

@@ -133,6 +133,14 @@ The format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 
 ### Fixed
 
+- **A2aEventQueue thread-safety** (#7604): Guarded `_subscriptions` and `_events` with a
+  `threading.Lock` to prevent `RuntimeError: dictionary changed size during iteration` when
+  `publish()`, `subscribe_local()`, and `unsubscribe()` are called concurrently from multiple
+  threads. The `publish()` method snapshots the subscriber dict inside the lock and invokes
+  callbacks outside the lock to prevent deadlocks. The `_is_closed` check in `publish()` is
+  performed inside the lock to eliminate a TOCTOU race with concurrent `close()` calls. The
+  `is_closed` property also reads `_is_closed` under the lock for memory-visibility guarantees.
+
 - **Validation Gate Empty-Run Guard** (#7508): Fixed `ApplyValidationSummary.all_required_passed`
   returning `True` when zero validations were run, silently bypassing the apply gate. The property
   now returns `False` when the validation result set is empty (`is_empty` is `True`), ensuring

CONTRIBUTORS.md

@@ -14,4 +14,5 @@ Below are some of the specific details of various contributions.
 
 * Jeffrey Phillips Freeman has acted as Lead Developer, daily contributor, and Project Owner.
 * Brent E. Edwards has contributed quality assurance, test coverage, and CI pipeline improvements.
+* HAL 9000 has contributed automated bug fixes, thread-safety improvements, and concurrency hardening including the A2aEventQueue threading.Lock guard (#7604).
 * This project was made possible thanks to considerable donation of time, money, and resources by CleverThis, Inc.