Buffered text/photo/media-group flushes and the polling-error recovery
task sit behind an asyncio.sleep(). On disconnect they kept running and
dispatched handle_message() into a torn-down session, producing stale or
duplicate deliveries. disconnect() only cancelled media-group and photo
batch tasks — text batches and the polling-error task leaked.
Set a _drop_delayed_deliveries flag from _mark_disconnected/_set_fatal_error
(cleared by _mark_connected) and check it in all enqueue+flush paths so a
flush that wins the race against teardown drops instead of dispatching.
_cancel_pending_delivery_tasks() now cancels+clears all four task maps,
skipping the current task. Media-group flush finally-block guarded so a
cancelled stale flush cannot erase a replacement task handle.
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