`container_backend_for_task` already catches everything inside `_terminal_env_type_for_task`
and `_uses_container_paths` and returns defaults, and every CheckpointManager has
`unsupported_backend_reason`: call the method directly (no getattr fallback), drop the
try/except in `unsupported_backend_reason` and the suppress() around the import+call in
turn_explainers (the one around record_agent_write stays). The rollback.restore fakes in the
server tests gain the method they now must provide.
1b083b85a3