fix(gateway): log error-notification failures instead of silently swallowing (#54472)

* fix(gateway): log error-notification failures instead of silently swallowing

The last-resort exception handler in _process_message_background() that
sends an error notice to the user caught all exceptions with a bare pass,
leaving zero trace when the notification itself failed. Upgrade to
logger.error(..., exc_info=True) so a failed error-notification send is
debuggable post-mortem.

Salvaged from #6499 by @BongSuCHOI (the logging-upgrade portion only).

* docs: add PR infographic for gateway error-notify logging

gateway/platforms/base.py

@@ -4955,8 +4955,11 @@ class BasePlatformAdapter(ABC):
                     ),
                     metadata=_thread_metadata,
                 )
-            except Exception:
-                pass  # Last resort — don't let error reporting crash the handler
+            except Exception as notify_err:
+                logger.error(
+                    "[%s] Failed to send error notification to user: %s",
+                    self.name, notify_err, exc_info=True,
+                )  # Last resort — don't let error reporting crash the handler
         finally:
             # Stop typing before any deferred callback work.  Post-delivery
             # callbacks may perform platform I/O; a stuck callback must not