When tasks created by a workflow execution complete or fail, the WorkflowExecution should transition to terminal status accordingly.

Currently, activate() creates an execution in RUNNING status and cancel_execution() transitions to CANCELLED, but there is no mechanism to advance an execution to COMPLETED or FAILED based on task outcomes.

What needs to happen

1. COMPLETED transition: When all TASK nodes in the execution have their corresponding tasks completed successfully, the execution should transition to COMPLETED with completed_at set.

2. FAILED transition: When any task fails (or a configurable failure policy is met), the execution should transition to FAILED with error and completed_at set.

3. Event constants: WORKFLOW_EXEC_COMPLETED and WORKFLOW_EXEC_FAILED are already defined in src/synthorg/observability/events/workflow_execution.py but are currently unused -- they should be used when logging these transitions.

Implementation options

• Polling: A periodic check that queries task statuses for running executions
• Event-driven: TaskEngine emits events on task completion/failure, and a listener updates the execution
• Callback: The workflow execution service registers a callback with the TaskEngine at activation time

Context

• Parent feature: feat: implement visual workflow editor #247 (visual workflow editor)
• Execution activation: feat: workflow execution -- instantiate tasks from WorkflowDefinition #1004 (implemented in PR feat: workflow execution -- instantiate tasks from WorkflowDefinition #1040)
• The completed_at cross-field validator was added in PR feat: workflow execution -- instantiate tasks from WorkflowDefinition #1040 -- terminal statuses now require completed_at to be set