#46 Enable and fix test_update_api.py - Temporal-style Update API

tests/integration/test_update_api.py is skipped (reason: Update API not fully implemented - workflow doesn't process updates). Need to: 1. Implement tools.testing.update_handler tool 2. Implement ctx.handle_update() in DurableContext 3. Add POST /workflows/{id}/updates endpoint with LISTEN/NOTIFY 4. Enable tests and ensure they pass Tests cover: - test_successful_update_processing: sync update with result - test_update_timeout: timeout handling - test_concurrent_updates: ordered concurrent update processing