After a transient Telegram 502, _handle_polling_network_error's
stop()+start_polling() cycle can leave PTB's Updater with `running=True`
but a wedged consumer task that never makes progress. No error_callback
fires in that state, so the reconnect ladder never advances past attempt
1, the MAX_NETWORK_RETRIES fatal-error path is never reached, and the
gateway sits silent indefinitely.
Schedule a heartbeat probe (60s after a successful reconnect) that
verifies Updater.running is still True and bot.get_me() responds within
a tight asyncio.wait_for timeout. Either failure feeds back into the
reconnect ladder so the existing escalation path fires.
No PTB-internal coupling, no Application rebuild — minimal additive
defense inside the existing reconnect abstraction.
Tests cover healthy / Updater non-running / probe timeout / probe
network error / already-fatal cases, plus an integration check that the
probe is actually scheduled after a successful start_polling().
Closes the silent-wedge case observed in the wild after a transient
Telegram 502; existing reconnect tests updated to mock bot.get_me() now
that the success path schedules a heartbeat probe.
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