fix(agent): ensure turns are sorted

Ensures that session turns retrieved from the agent persistence layer are
sorted by their `started_at` timestamp, as the key-value store does not
guarantee order.

Closes #2852

## Test Plan

- [ ] Add unit tests

llama_stack/providers/inline/agents/meta_reference/persistence.py

@@ -128,6 +128,11 @@ class AgentPersistence:
             except Exception as e:
                 log.error(f"Error parsing turn: {e}")
                 continue
+
+        # The kvstore does not guarantee order, so we sort by started_at
+        # to ensure consistent ordering of turns.
+        turns.sort(key=lambda t: t.started_at)
+
         return turns
 
     async def get_session_turn(self, session_id: str, turn_id: str) -> Turn | None: