fix(gateway): route /background through active-session bypass

When /background was sent during an active run, it was not in the
platform adapter's bypass list and fell through to the interrupt path
instead of spawning a parallel background task.

Add "background" to the active-session command bypass in the platform
adapter, and add an early return in the gateway runner's running-agent
guard to route /background to _handle_background_command() before it
reaches the default interrupt logic.

Fixes #6827

gateway/platforms/base.py

@@ -1303,7 +1303,7 @@ class BasePlatformAdapter(ABC):
             # session lifecycle and its cleanup races with the running task
             # (see PR #4926).
             cmd = event.get_command()
-            if cmd in ("approve", "deny", "status", "stop", "new", "reset"):
+            if cmd in ("approve", "deny", "status", "stop", "new", "reset", "background"):
                 logger.debug(
                     "[%s] Command '/%s' bypassing active-session guard for %s",
                     self.name, cmd, session_key,

gateway/run.py

@@ -1991,6 +1991,11 @@ class GatewayRunner:
                     return await self._handle_approve_command(event)
                 return await self._handle_deny_command(event)
 
+            # /background must bypass the running-agent guard — it starts a
+            # parallel task and must never interrupt the active conversation.
+            if _cmd_def_inner and _cmd_def_inner.name == "background":
+                return await self._handle_background_command(event)
+
             if event.message_type == MessageType.PHOTO:
                 logger.debug("PRIORITY photo follow-up for session %s — queueing without interrupt", _quick_key[:20])
                 adapter = self.adapters.get(source.platform)