fix(channels): prevent retry amplification and silent message loss across channels

Audited all channel implementations for overly broad exception handling
that causes retry amplification or silent message loss during network
errors. This is the same class of bug as #3050 (Telegram _send_text).

Fixes by channel:

Telegram (send_delta):
- _stream_end path used except Exception for HTML edit fallback
- Network errors (TimedOut, NetworkError) triggered redundant plain
  text edit, doubling connection demand during pool exhaustion
- Changed to except BadRequest, matching the _send_text fix

Discord:
- send() caught all exceptions without re-raising
- ChannelManager._send_with_retry() saw successful return, never retried
- Messages silently dropped on any send failure
- Added raise after error logging

DingTalk:
- _send_batch_message() returned False on all exceptions including
  network errors — no retry, fallback text sent unnecessarily
- _read_media_bytes() and _upload_media() swallowed transport errors,
  causing _send_media_ref() to cascade through doomed fallback attempts
- Added except httpx.TransportError handlers that re-raise immediately

WeChat:
- Media send failure triggered text fallback even for network errors
- During network issues: 3×(media + text) = 6 API calls per message
- Added specific catches: TimeoutException/TransportError re-raise,
  5xx HTTPStatusError re-raises, 4xx falls back to text

QQ:
- _send_media() returned False on all exceptions
- Network errors triggered fallback text instead of retry
- Added except (aiohttp.ClientError, OSError) that re-raises

Tests: 331 passed (283 existing + 48 new across 5 channel test files)

Fixes: #3054
Related: #3050, #3053

nanobot/channels/dingtalk.py

@@ -337,6 +337,9 @@ class DingTalkChannel(BaseChannel):
                 content_type = (resp.headers.get("content-type") or "").split(";")[0].strip()
                 filename = self._guess_filename(media_ref, self._guess_upload_type(media_ref))
                 return resp.content, filename, content_type or None
+            except httpx.TransportError as e:
+                logger.error("DingTalk media download network error ref={} err={}", media_ref, e)
+                raise
             except Exception as e:
                 logger.error("DingTalk media download error ref={} err={}", media_ref, e)
                 return None, None, None
@@ -388,6 +391,9 @@ class DingTalkChannel(BaseChannel):
                 logger.error("DingTalk media upload missing media_id body={}", text[:500])
                 return None
             return str(media_id)
+        except httpx.TransportError as e:
+            logger.error("DingTalk media upload network error type={} err={}", media_type, e)
+            raise
         except Exception as e:
             logger.error("DingTalk media upload error type={} err={}", media_type, e)
             return None
@@ -437,6 +443,9 @@ class DingTalkChannel(BaseChannel):
                 return False
             logger.debug("DingTalk message sent to {} with msgKey={}", chat_id, msg_key)
             return True
+        except httpx.TransportError as e:
+            logger.error("DingTalk network error sending message msgKey={} err={}", msg_key, e)
+            raise
         except Exception as e:
             logger.error("Error sending DingTalk message msgKey={} err={}", msg_key, e)
             return False