## Why
Transient connection and registry failures can reach remote filesystem callers as generic `io::ErrorKind::Other` errors. Callers need a typed signal to identify failures that read-only preparation can retry.
## What changed
- Add `ExecServerError::is_retryable_preparation_error()` to classify transient registry, connection, handshake, timeout, and session-attachment failures, including errors wrapped in `ConnectionAttempt`.
- Map these failures to `io::ErrorKind::BrokenPipe` in the remote filesystem adapter.
- Keep authentication, TLS, and malformed protocol failures outside the retryable classification; error text alone does not trigger it.
## Testing
Add a regression test showing that a wrapped registry service-unavailable error maps to `BrokenPipe`, while a protocol error containing transport-closed text remains `Other`.
GitOrigin-RevId: c3e1bbab8f977a0b421eba5d6d3071ada72c09ff
mtsui-oai
c847294c60