fix(telegram): narrow exception catch in _send_text to prevent retry amplification

Previously _send_text() caught all exceptions (except Exception) when
sending HTML-formatted messages, falling back to plain text even for
network errors like TimedOut and NetworkError. This caused connection
demand to double during pool exhaustion scenarios (3 retries × 2
fallback attempts = 6 calls per message instead of 3).

Now only catches BadRequest (HTML parse errors), letting network errors
propagate immediately to the retry layer where they belong.

Fixes: HKUDS/nanobot#3050

nanobot/channels/telegram.py

@@ -520,7 +520,10 @@ class TelegramChannel(BaseChannel):
                 reply_parameters=reply_params,
                 **(thread_kwargs or {}),
             )
-        except Exception as e:
+        except BadRequest as e:
+            # Only fall back to plain text on actual HTML parse/format errors.
+            # Network errors (TimedOut, NetworkError) should propagate immediately
+            # to avoid doubling connection demand during pool exhaustion.
             logger.warning("HTML parse failed, falling back to plain text: {}", e)
             try:
                 await self._call_with_retry(