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/matrix.py

@@ -412,6 +412,7 @@ class MatrixChannel(BaseChannel):
         try:
             response = await self.client.content_repository_config()
         except Exception:
+            self.logger.warning("Failed to fetch server upload limit", exc_info=True)
             return None
         upload_size = getattr(response, "upload_size", None)
         if isinstance(upload_size, int) and upload_size > 0:
@@ -457,6 +458,7 @@ class MatrixChannel(BaseChannel):
                     filesize=size_bytes,
                 )
         except Exception:
+            self.logger.warning("Media upload failed", exc_info=True)
             return fail
 
         upload_response = upload_result[0] if isinstance(upload_result, tuple) else upload_result
@@ -476,6 +478,7 @@ class MatrixChannel(BaseChannel):
         try:
             await self._send_room_content(room_id, content)
         except Exception:
+            self.logger.warning("Failed to send room content to room_id=%s", room_id, exc_info=True)
             return fail
         return None
 
@@ -553,8 +556,8 @@ class MatrixChannel(BaseChannel):
                     # we are editing the same message all the time, so only the first time the event id needs to be set
                     buf.event_id = response.event_id
             except Exception:
+                self.logger.warning("Stream send/edit failed for chat_id=%s", chat_id, exc_info=True)
                 await self._stop_typing_keepalive(chat_id, clear_typing=True)
-                pass
 
 
     def _register_event_callbacks(self) -> None:

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: