Add backend capability cache reset helper for jail service tests

Docs/Tasks.md

@@ -466,6 +466,8 @@ Imports for side effects are an anti-pattern that creates hidden startup depende
 
 ### Task 16 — Reset _backend_cmd_supported between test runs
 
+**Status:** Completed
+
 **Severity:** Medium
 
 **Where:**

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
 # ---------------------------------------------------------------------------

backend/tests/test_services/test_jail_service.py

@@ -192,7 +192,7 @@ class TestListJails:
         fail2ban log.
         """
         # Reset the capability cache to test detection.
-        jail_service._backend_cmd_supported = None
+        await jail_service._reset_backend_capability_cache()
 
         responses = {
             "status": _make_global_status("sshd"),
@@ -217,7 +217,7 @@ class TestListJails:
     async def test_backend_idle_commands_supported(self) -> None:
         """list_jails detects and sends backend/idle commands when supported."""
         # Reset the capability cache to test detection.
-        jail_service._backend_cmd_supported = None
+        await jail_service._reset_backend_capability_cache()
 
         responses = {
             "status": _make_global_status("sshd"),
@@ -243,7 +243,7 @@ class TestListJails:
     async def test_backend_idle_commands_cached_after_first_probe(self) -> None:
         """list_jails caches capability result and reuses it across polling cycles."""
         # Reset the capability cache.
-        jail_service._backend_cmd_supported = None
+        await jail_service._reset_backend_capability_cache()
 
         responses = {
             "status": _make_global_status("sshd, nginx"),