Refresh stack child head guards after collapse

[cbusillo]

Summary

• update stack child close guards after each collapse merge advances an intermediate child branch
• keep leaf child guards unchanged when their own head was not mutated
• cover the three-PR stack case where the intermediate child must close against the merge commit SHA

Validation

• uv run --extra dev ruff check control_plane/contracts/merge_train_stack_collapse.py tests/test_merge_train_stack_collapse.py
• uv run python -m unittest tests.test_merge_train_stack_collapse tests.test_service.LaunchplaneServiceTests.test_merge_train_batch_landing_service_closes_stack_children_after_root_lands
• uv run python -m unittest tests.test_merge_train_stack_collapse tests.test_merge_train_github tests.test_service