fix: complete ensure_ascii=False and UTF-8 encoding migration

nanobot/session/manager.py

@@ -110,7 +110,7 @@ class SessionManager:
             if legacy_path.exists():
                 import shutil
                 shutil.move(str(legacy_path), str(path))
-                logger.info(f"Migrated session {key} from legacy path")
+                logger.info("Migrated session {} from legacy path", key)
 
         if not path.exists():
             return None
@@ -144,7 +144,7 @@ class SessionManager:
                 last_consolidated=last_consolidated
             )
         except Exception as e:
-            logger.warning(f"Failed to load session {key}: {e}")
+            logger.warning("Failed to load session {}: {}", key, e)
             return None
     
     def save(self, session: Session) -> None:
@@ -163,7 +163,6 @@ class SessionManager:
             for msg in session.messages:
                 f.write(json.dumps(msg, ensure_ascii=False) + "\n")
 
-
         self._cache[session.key] = session
     
     def invalidate(self, key: str) -> None: