Refactor workflow agent and routing components for improved functionality

- Introduce WorkflowAgentStage to manage agent stage configurations and enhance interaction with the workflow engine.
- Implement WorkflowEdgeEvaluator for priority-aware edge evaluation, improving routing decisions based on conditions and user turns.
- Update WorkflowBrain to handle user turns and routing more effectively, ensuring agents cannot have only one default path.
- Enhance CallEndCoordinator to track speech events and manage call termination based on queued speech.
- Add new models and output handling for workflow interactions, improving clarity and maintainability.
- Update tests to validate the new routing logic and agent behavior under various scenarios.

backend/services/workflow_engine.py

@@ -177,6 +177,11 @@ class WorkflowEngine:
             try:
                 if operator == "exists":
                     matched = exists if expected is not False else not exists
+                elif not exists:
+                    # Missing values are never equal, unequal, greater or
+                    # contained. Use the explicit ``exists`` operator when the
+                    # distinction between missing and present is important.
+                    matched = False
                 elif operator == "eq":
                     matched = actual == expected
                 elif operator == "neq":