fix: remove unused sqlalchemy.exists import in storage/search.py

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: acailic

storage/search.py

@@ -10,7 +10,7 @@
 from datetime import datetime
 from typing import Any
 
-from sqlalchemy import String, cast, exists, or_, select
+from sqlalchemy import String, cast, or_, select
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from agent_debugger_sdk.core.events import Session, TraceEvent