Both polling reconnect paths end on the same 'health pending getUpdates
progress' line, and _record_polling_progress completed silently — so the
log stream for 'reconnected and healthy' was byte-identical to
'reconnected and hung', and a wedged long-poll (#87057 / #69314 /
#71239 class) stayed invisible until a user noticed silence. The only
detection method was sending the bot a test message (#90504).
Emit one INFO on the first confirmed getUpdates round-trip of each
generation, inside the existing event-set branch so steady-state polling
adds no log volume. This turns the pending line into a resolvable pair
('health pending' -> 'confirmed healthy') whose absence after a
reconnect is a reliable hung-poll signature.
Fixes#90504
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