Add backend capability cache reset helper for jail service tests

backend/app/services/jail_service.py

@@ -248,6 +248,18 @@ async def _check_backend_cmd_supported(
         return _backend_cmd_supported
 
 
+async def _reset_backend_capability_cache() -> None:
+    """Reset the cached backend/idle capability detection state.
+
+    This helper is intended for test isolation and for any scenario where the
+    cached probe result must be invalidated before the next detection attempt.
+    """
+    global _backend_cmd_supported
+
+    async with _backend_cmd_lock:
+        _backend_cmd_supported = None
+
+
 # ---------------------------------------------------------------------------
 # Public API — Jail listing & detail
 # ---------------------------------------------------------------------------