The _session_expiry_watcher retried failed memory flushes forever
because exceptions were caught at debug level without setting
memory_flushed=True. Expired sessions with transient failures
(rate limits, network errors) would retry every 5 minutes
indefinitely, burning API quota and blocking gateway message
processing via 429 rate limit cascades.
Observed case: a March 19 session retried 28+ times over ~17 days,
causing repeated 429 errors that made Telegram unresponsive.
Add a per-session failure counter (_flush_failures) that gives up
after 3 consecutive attempts and marks the session as flushed to
break the loop.
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