fix: raise instead of swallowing on outbound-message path errors

Per reviewer request (chengyongru): raise exceptions on the outbound
message path so ChannelManager can trigger retry logic, matching the
pattern from commit 98c2f7cc (Weixin channel cleanup).

Changes:
- _resolve_server_upload_limit_bytes: warning → error (non-fatal config)
- _upload_and_send_attachment media upload: raise instead of swallow
- _upload_and_send_attachment room send: raise instead of swallow
- send_delta stream edit: error + raise after cleanup
- weixin _load_state: warning → error (non-fatal state load)

nanobot/channels/weixin.py

@@ -208,7 +208,7 @@ class WeixinChannel(BaseChannel):
                 self.config.base_url = base_url
             return bool(self._token)
         except Exception:
-            self.logger.warning("Failed to load Weixin account state", exc_info=True)
+            self.logger.error("Failed to load Weixin account state", exc_info=True)
             return False
 
     def _save_state(self) -> None: