fix(gateway): let /btw dispatch mid-turn instead of being rejected

/btw spawns a parallel ephemeral side-question task (self-guarded against
concurrent /btw on the same chat) — exactly like /background. But it was
missing from the running-agent bypass list in _handle_message(), so it
fell through to the catch-all and returned:

  ⏳ Agent is running — /btw can't run mid-turn. Wait for the current
  response or /stop first.

That's the opposite of what /btw is for — asking a side question while
the main turn is still working. Add the bypass next to /background and a
regression test covering the mid-turn dispatch path.

Reported by @IuriiTiunov on Telegram.

gateway/run.py

@@ -3501,6 +3501,14 @@ class GatewayRunner:
             if _cmd_def_inner and _cmd_def_inner.name == "background":
                 return await self._handle_background_command(event)
 
+            # /btw must bypass the running-agent guard for the same reason
+            # as /background: it spawns a parallel ephemeral side-question
+            # task (see _handle_btw_command) that doesn't interrupt the
+            # active conversation and self-guards against concurrent /btw
+            # on the same chat.
+            if _cmd_def_inner and _cmd_def_inner.name == "btw":
+                return await self._handle_btw_command(event)
+
             # Session-level toggles that are safe to run mid-agent —
             # /yolo can unblock a pending approval prompt, /verbose cycles
             # the tool-progress display mode for the ongoing stream.