- probe_only is consumed inside __init__ only; no instance attribute.
- _sync_manager is assigned only on the probe branch, after the connection is
up, so a normal SSHEnvironment whose constructor fails still behaves exactly
as before (its __del__ cleanup does not reach the shared socket teardown).
- _remote_home has no reader on the probe path; not assigned.
- prompt_builder passes probe_only=True unconditionally: which backends honor
it is the builder table's decision, not a caller-side env_type check.
- Tests trimmed to the probe_only contract: the cleanup-raises case was green
on main (pre-existing try/except), the socket-name length and double-cleanup
assertions covered pre-existing behaviour.
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