Enhance conversation history and runtime variable management

- Update ConversationRecorder to include source and nodeId metadata in transcripts for better context tracking.
- Introduce optional variable handling in DynamicVariableStore, allowing for unset variables to be rendered as empty without raising errors.
- Refactor WorkflowBrain to apply turn configurations and manage interaction policies dynamically, improving agent responsiveness.
- Implement tests to ensure proper handling of updated session variables and workflow metadata in various scenarios.

backend/services/conversation_history.py

@@ -77,6 +77,10 @@ class ConversationRecorder:
             role = str(message.get("role") or "")
             content = str(message.get("content") or "").strip()
             event_key = f"transcript:{role}:{timestamp}:{content}"
+            if message.get("source"):
+                extra["source"] = str(message["source"])
+            if message.get("nodeId"):
+                extra["node_id"] = str(message["nodeId"])
         elif event_type == "assistant-text-end":
             role = "assistant"
             content = str(message.get("content") or "").strip()