fix(gateway): correct misleading log text for unknown /commands

The warning said 'forwarding as plain text' but the code returns a
user-facing error reply instead of forwarding. Describe what actually
happens.

gateway/run.py

@@ -2177,7 +2177,7 @@ class GatewayRunner:
                     if command.replace("_", "-") not in GATEWAY_KNOWN_COMMANDS:
                         logger.warning(
                             "Unrecognized slash command /%s from %s — "
-                            "forwarding as plain text",
+                            "replying with unknown-command notice",
                             command,
                             source.platform.value if source.platform else "?",
                         )