fix(telegram): self-reschedule reconnect when start_polling fails (#3268)

After a Telegram 502, _handle_polling_network_error calls updater.stop()
then start_polling(). If start_polling() also raises, the old code logged
a warning and returned — but the comment 'The next network error will
trigger another attempt' was wrong. The updater loop is dead after stop(),
so no further error callbacks ever fire. The gateway stays alive but
permanently deaf to messages.

Fix: when start_polling() fails in the except branch, schedule a new
_handle_polling_network_error task to continue the exponential backoff
retry chain. The task is tracked in _background_tasks (preventing GC).
Guarded by has_fatal_error to avoid spurious retries during shutdown.

Closes #3173.
Salvaged from PR #3177 by Mibayy.

gateway/platforms/telegram.py

@@ -219,7 +219,14 @@ class TelegramAdapter(BasePlatformAdapter):
             self._polling_network_error_count = 0
         except Exception as retry_err:
             logger.warning("[%s] Telegram polling reconnect failed: %s", self.name, retry_err)
-            # The next network error will trigger another attempt.
+            # start_polling failed — polling is dead and no further error
+            # callbacks will fire, so schedule the next retry ourselves.
+            if not self.has_fatal_error:
+                task = asyncio.ensure_future(
+                    self._handle_polling_network_error(retry_err)
+                )
+                self._background_tasks.add(task)
+                task.add_done_callback(self._background_tasks.discard)
 
     async def _handle_polling_conflict(self, error: Exception) -> None:
         if self.has_fatal_error and self.fatal_error_code == "telegram_polling_conflict":