Retire landed merge train batches

[cbusillo]

Summary

• stop the controller from reusing a passed candidate after the latest landing plan has fully merged
• return a terminal batch_landed action for completed landing plans
• add regression coverage that a post-landing controller tick does not create another landing plan

Tests

• uv run python -m unittest tests.test_service.LaunchplaneServiceTests.test_merge_train_controller_advances_unstacked_batch_flow
• uv run python -m unittest tests.test_service.LaunchplaneServiceTests.test_merge_train_controller_stops_after_candidate_failure
• uv run python -m unittest