Fixes a race condition where the namespace filter was applied outside
the threading.RLock, allowing concurrent mutations (discover(), clear())
to corrupt the iteration state. The filter now runs inside the locked
section, matching the locking discipline of all other public methods.

- Moved namespace filtering inside lock in list_actors()
- Added BDD concurrency regression test
- Added unit test for thread-safety under concurrent discover/clear
- Updated CHANGELOG.md with fix description
- Updated CONTRIBUTORS.md

@@ -0,0 +9,4 @@
+import tempfile
+import threading
+import time
+from contextlib import ExitStack

@@ -0,0 +13,4 @@
+from pathlib import Path
+from typing import Any
+
+import yaml

@@ -0,0 +17,4 @@
+from behave import given, then, when  # type: ignore[attr-defined]
+
+from cleveragents.actor.loader import ActorLoader
+from cleveragents.core.exceptions import ValidationError

@@ -0,0 +1,36 @@
+@tdd_issue @issue_8660

@@ -0,0 +13,4 @@
+import time
+from pathlib import Path
+
+import yaml

@@ -0,0 +115,4 @@
+        def _list_worker() -> None:
+            try:
+                for _ in range(30):
+                    all_actors = loader.list_actors()