update variagble name

Author: Nicole Serafino

agentic_ai/agents/agent_service/single_agent/agent.py

@@ -257,7 +257,7 @@ def __init__(self, state_store: Dict[str, Any], session_id: str) -> None:
             logging.info(f"Found stored conversation ID: {self.conversation_id}")
 
         # Get connection string from environment variable
-        self.conn_str = os.environ.get("PROJECT_CONNECTION_STRING")
+        self.conn_str = os.environ.get("AZURE_AI_AGENT_PROJECT_CONNECTION_STRING")
         if not self.conn_str:
             raise ValueError("PROJECT_CONNECTION_STRING environment variable is not set")