After sustained Bad Gateway / TimedOut reconnect cycles, the PTB httpx
client can enter a state where bot.send_message() returns a valid
Message (real message_id) but the message never reaches the recipient.
TelegramAdapter.send returns SendResult(success=True) and cron's
live-adapter branch marks the run delivered while the message is
silently dropped.
Add a _send_path_degraded flag. _handle_polling_network_error sets it
on reconnect storms; the existing _verify_polling_after_reconnect
heartbeat probe clears it once getMe() confirms the Bot client is
healthy. While the flag is set, send() short-circuits with
SendResult(success=False, retryable=True) so cron falls through to
the standalone delivery path (fresh HTTP session).
Closes#31165.
Co-authored-by: teknium1 <127238744+teknium1@users.noreply.github.com>
