fix(telegram): fall back to no thread_id on 'Message thread not found' (#3390)

python-telegram-bot's BadRequest inherits from NetworkError, so the
send() retry loop was catching 'Message thread not found' as a transient
network error and retrying 3 times before silently failing. This killed
all tool progress messages, streaming responses, and typing indicators
when the incoming message carried an invalid message_thread_id.

Now detect BadRequest inside the NetworkError handler:
- 'thread not found' + thread_id set → clear thread_id and retry once
  (message still reaches the chat, just without topic threading)
- Other BadRequest errors → raise immediately (permanent, don't retry)
- True NetworkError → retry as before (transient)

252 silent failures in gateway.log traced to this on 2026-03-26.

5 new tests for thread fallback, non-thread BadRequest, no-thread sends,
network retry, and multi-chunk fallback.

gateway/platforms/telegram.py

@@ -707,9 +707,15 @@ class TelegramAdapter(BasePlatformAdapter):
             except ImportError:
                 _NetErr = OSError  # type: ignore[misc,assignment]
 
+            try:
+                from telegram.error import BadRequest as _BadReq
+            except ImportError:
+                _BadReq = None  # type: ignore[assignment,misc]
+
             for i, chunk in enumerate(chunks):
                 should_thread = self._should_thread_reply(reply_to, i)
                 reply_to_id = int(reply_to) if should_thread else None
+                effective_thread_id = int(thread_id) if thread_id else None
 
                 msg = None
                 for _send_attempt in range(3):
@@ -721,7 +727,7 @@ class TelegramAdapter(BasePlatformAdapter):
                                 text=chunk,
                                 parse_mode=ParseMode.MARKDOWN_V2,
                                 reply_to_message_id=reply_to_id,
-                                message_thread_id=int(thread_id) if thread_id else None,
+                                message_thread_id=effective_thread_id,
                             )
                         except Exception as md_error:
                             # Markdown parsing failed, try plain text
@@ -733,12 +739,30 @@ class TelegramAdapter(BasePlatformAdapter):
                                     text=plain_chunk,
                                     parse_mode=None,
                                     reply_to_message_id=reply_to_id,
-                                    message_thread_id=int(thread_id) if thread_id else None,
+                                    message_thread_id=effective_thread_id,
                                 )
                             else:
                                 raise
                         break  # success
                     except _NetErr as send_err:
+                        # BadRequest is a subclass of NetworkError in
+                        # python-telegram-bot but represents permanent errors
+                        # (not transient network issues). Detect and handle
+                        # specific cases instead of blindly retrying.
+                        if _BadReq and isinstance(send_err, _BadReq):
+                            err_lower = str(send_err).lower()
+                            if "thread not found" in err_lower and effective_thread_id is not None:
+                                # Thread doesn't exist — retry without
+                                # message_thread_id so the message still
+                                # reaches the chat.
+                                logger.warning(
+                                    "[%s] Thread %s not found, retrying without message_thread_id",
+                                    self.name, effective_thread_id,
+                                )
+                                effective_thread_id = None
+                                continue
+                            # Other BadRequest errors are permanent — don't retry
+                            raise
                         if _send_attempt < 2:
                             wait = 2 ** _send_attempt
                             logger.warning("[%s] Network error on send (attempt %d/3), retrying in %ds: %s",