Telegram polling entered a self-inflicted ~31s loop of 409 Conflict ->
retry -> resume -> Conflict. The error_callback PTB invokes synchronously
inside its internal network_retry_loop only scheduled our async recovery
task (loop.create_task) and returned, so PTB kept polling getUpdates on its
own while our handler concurrently ran stop -> sleep -> start_polling. The
two polling sessions overlapped and Telegram returned a fresh 409.
Fix: in the conflict branch of the error_callback, synchronously set PTB's
private polling stop_event before scheduling recovery. PTB's loop exits on
its next tick (it races that event in do_action), so our handler owns
polling alone. The handler's await updater.stop() drains the task and PTB
clears the event, so the subsequent start_polling() builds a fresh event
and is not poisoned.
Keeps the existing reconnect ladder intact (option B) — fixes only the
race. Defensive: probes mangled + unmangled stop_event spellings and no-ops
(prior behaviour) if neither exists; never flips _running, which would make
the handler skip stop() and leave the loop wedged.
f03823014b