_fetch_new_messages() wrapped the whole IMAP connect/login/select/search/
fetch sequence in a bare except that logged and returned an empty list —
indistinguishable from a genuinely empty inbox. The adapter never invoked
its fatal-error handler, so the gateway's reconnect/backoff/status
machinery never learned the mailbox was unreachable; outages lasted until
a manual restart.
Track fetch failure on the adapter and, when the poll loop observes it,
set a retryable fatal error (email_imap_fetch_failed) and notify the
gateway handler so the platform enters the reconnect queue just like a
startup connection failure.
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