The knob's tests drove _start_polling_mode only, so a policy regression on
the webhook start path went undetected. The flag is not a no-op there: PTB
feeds it to delete_webhook/set_webhook, which drop Telegram's stored updates.
Asserts the cold-boot flag follows the knob, that the decision is logged, and
that a watcher reconnect still preserves the queue with the knob at its
default (the #46621 guarantee).
43b3fa96b2