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/brains/base.py

@@ -60,6 +60,9 @@ class BrainRuntime:
     ) = None
     set_knowledge_scope: Callable[[dict[str, Any]], None] | None = None
     set_input_enabled: Callable[[bool], None] | None = None
+    apply_turn_config: (
+        Callable[[bool, dict[str, Any]], Awaitable[None]] | None
+    ) = None
     flow_global_functions: list[Any] = field(default_factory=list)