A platform with a missing dependency or missing credentials can never
succeed on retry, but connect() returned bare False, so the gateway
treated the failure as transient and queued it for background
reconnection — looping forever at the backoff cap. Set
_set_fatal_error(..., retryable=False) for missing-dependency and
missing-credential failures in the Slack, Telegram, and Discord
adapters so the reconnect watcher drops them from the retry queue.
Salvaged from PR #31057 by @dskwe (reapplied onto the plugin-migrated
adapter paths). Fixes#31049.
