address P2 feedback: add lock docstring warning, remove redundant mock write

- Add WARNING docstring to _get_shared_session_lock() about not resetting
  the lock to None while coroutines may be in the recovery path
- Remove redundant proxy_server_module.shared_aiohttp_session assignment
  in mock_init (add_shared_session_to_data overwrites it synchronously)

litellm/proxy/route_llm_request.py

@@ -128,7 +128,12 @@ _shared_session_lock: Optional[asyncio.Lock] = None
 
 
 def _get_shared_session_lock() -> asyncio.Lock:
-    """Lazily create the shared session lock (must be called within a running event loop)."""
+    """Lazily create the shared session lock (must be called within a running event loop).
+
+    WARNING: Do not reset _shared_session_lock to None while any coroutine may be
+    executing the session-recovery path; doing so breaks the double-checked locking
+    guarantee and can cause duplicate session creation.
+    """
     global _shared_session_lock
     if _shared_session_lock is None:
         _shared_session_lock = asyncio.Lock()

tests/test_litellm/proxy/test_aiohttp_session_recovery.py

@@ -161,7 +161,6 @@ async def test_add_shared_session_concurrent_recreation_uses_lock():
         call_count += 1
         # Simulate some async work
         await asyncio.sleep(0.01)
-        proxy_server_module.shared_aiohttp_session = new_session
         return new_session
 
     with patch.object(