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/node_specs.py

@@ -162,6 +162,8 @@ def _normalize_settings(settings: dict[str, Any], *, global_prompt: str = "") ->
     settings.setdefault("knowledgeMode", "automatic")
     settings.setdefault("knowledgeTopN", 5)
     settings.setdefault("knowledgeScoreThreshold", 0.0)
+    settings.setdefault("enableInterrupt", True)
+    settings.setdefault("turnConfig", {})
 
 
 def normalize_graph(graph: dict[str, Any] | None) -> dict[str, Any]: