Add container-instance to _STOPPABLE_TYPES so that agents resource stop
accepts both container-instance and devcontainer-instance resources, as
required by the specification. The stop_container function already uses
docker stop with the container_id from the lifecycle tracker, which works
generically for both resource types.
- Add container-instance to _STOPPABLE_TYPES frozenset
- Update resource_stop() docstring to reflect both stoppable types
- Update devcontainer_cleanup.feature: F19 scenario now expects success
for container-instance stop (was incorrectly expecting rejection)
- Add TDD feature file and step definitions for issue #2588 regression
guard
ISSUES CLOSED: #2588
