Refactor memory metadata index to use Rust backend

Replaces the Python implementation of MemoryMetadataIndex with a Rust-accelerated version, removing legacy code and fallback logic. Updates search, add, and stats methods to delegate to the Rust backend. Also adds missing imports in llm_data_model.py and base_event.py for improved type support and event handling.

src/plugin_system/base/base_event.py

@@ -2,6 +2,7 @@ import asyncio
 from typing import Any
 
 from src.common.logger import get_logger
+from src.plugin_system.base.base_events_handler import BaseEventHandler
 
 logger = get_logger("base_event")