fix: log errors in silent exception handlers (matrix + weixin channels)

The Matrix channel had 4 bare except blocks that silently swallowed
transport errors with no logging — stream send/edit failures, media
upload failures, server config fetch failures, and room content send
failures. The Weixin channel had 1 silent state-load failure.

This mirrors commit 98c2f7cc ('fix(weixin): raise exceptions instead
of silently dropping messages') for the Matrix channel and adds a
warning for the remaining silent catch in Weixin's _load_state.

All failures now log at warning level with exc_info=True so operators
can diagnose intermittent Matrix/Weixin transport issues.

nanobot/channels/weixin.py

@@ -208,6 +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)
             return False
 
     def _save_state(self) -> None: