feat(memory): add rewound kwarg to on_session_switch hook

agent/memory_manager.py

@@ -491,6 +491,7 @@ class MemoryManager:
         *,
         parent_session_id: str = "",
         reset: bool = False,
+        rewound: bool = False,
         **kwargs,
     ) -> None:
         """Notify all providers that the agent's session_id has rotated.
@@ -503,6 +504,10 @@ class MemoryManager:
         per-session state so subsequent writes land in the correct
         session's record. See ``MemoryProvider.on_session_switch`` for
         the full contract.
+
+        ``rewound=True`` signals that session_id is unchanged but the
+        transcript was truncated; providers caching per-turn document
+        state should invalidate.
         """
         if not new_session_id:
             return
@@ -512,6 +517,7 @@ class MemoryManager:
                     new_session_id,
                     parent_session_id=parent_session_id,
                     reset=reset,
+                    rewound=rewound,
                     **kwargs,
                 )
             except Exception as e: