#109 Missing integration tests for ActivityOperator and auto-activity conversion

Issue #104 implemented _execute_as_activity() but no tests exist to verify: 1. Basic ActivityOperator execution 2. Activity success/completion events 3. Activity failure and retry logic 4. Auto-activity conversion (timeout > 30s) 5. Activity idempotency (checkpoint prevents double-queueing) 6. Workflow resumption after activity completion Current state: Full infrastructure implemented, zero test coverage.