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/common/data_models/llm_data_model.py

@@ -1,6 +1,8 @@
 from dataclasses import dataclass
 from typing import TYPE_CHECKING, Any
 
+from src.llm_models.payload_content.tool_option import ToolCall
+
 from . import BaseDataModel
 
 if TYPE_CHECKING: