Network errors through proxies (e.g. sing-box) can leave httpx
connections in a half-closed state occupying pool slots. After enough
reconnect cycles the 256-connection default fills up entirely, causing
Pool timeout: All connections in the connection pool are occupied.
Fix: cycle only the getUpdates request object (_request[0]) via
shut-down + re-initialize before restarting polling. This drains stale
connections without touching the general request (_request[1]) that
concurrent send_message / edit_message calls rely on.
The drain is applied to both _handle_polling_network_error and
_handle_polling_conflict reconnect paths via a shared
_drain_polling_connections() helper. Failures in the drain are
swallowed so reconnect always proceeds.
Based on #16466 by @Mirac1eSky.
d6137453ac