CheckpointManager.create_checkpoint() computed sandbox_path from
sandbox.context.sandbox_path but never stored it in the metadata dict.
This caused rollback_to() to always find metadata.get('sandbox_path')
returning None, silently skip the rollback, and return False.
The fix adds sandbox_path to the metadata dict immediately after it is
resolved from the sandbox context, before the SandboxCheckpoint is
constructed. rollback_to() can now retrieve the path and correctly
restore the sandbox filesystem state.
Added two new BDD scenarios to checkpoint_manager_coverage.feature:
- Verifies sandbox_path is automatically stored in metadata on create
- Verifies rollback succeeds without manually supplying sandbox_path
ISSUES CLOSED: #7488
HAL9000