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/weixin.py

@@ -985,7 +985,43 @@ class WeixinChannel(BaseChannel):
             for media_path in (msg.media or []):
                 try:
                     await self._send_media_file(msg.chat_id, media_path, ctx_token)
+                except (httpx.TimeoutException, httpx.TransportError) as net_err:
+                    # Network/transport errors: do NOT fall back to text —
+                    # the text send would also likely fail, and the outer
+                    # except will re-raise so ChannelManager retries properly.
+                    logger.error(
+                        "Network error sending WeChat media {}: {}",
+                        media_path,
+                        net_err,
+                    )
+                    raise
+                except httpx.HTTPStatusError as http_err:
+                    status_code = (
+                        http_err.response.status_code
+                        if http_err.response is not None
+                        else 0
+                    )
+                    if status_code >= 500:
+                        # Server-side / retryable HTTP error — same as network.
+                        logger.error(
+                            "Server error ({} {}) sending WeChat media {}: {}",
+                            status_code,
+                            http_err.response.reason_phrase
+                            if http_err.response is not None
+                            else "",
+                            media_path,
+                            http_err,
+                        )
+                        raise
+                    # 4xx client errors are NOT retryable — fall back to text.
+                    filename = Path(media_path).name
+                    logger.error("Failed to send WeChat media {}: {}", media_path, http_err)
+                    await self._send_text(
+                        msg.chat_id, f"[Failed to send: {filename}]", ctx_token,
+                    )
                 except Exception as e:
+                    # Non-network errors (format, file-not-found, etc.):
+                    # notify the user via text fallback.
                     filename = Path(media_path).name
                     logger.error("Failed to send WeChat media {}: {}", media_path, e)
                     # Notify user about failure via text