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.

tests/gateway/test_running_agent_session_toggles.py

@@ -165,3 +165,27 @@ async def test_reasoning_rejected_mid_run():
     assert result is not None
     assert "can't run mid-turn" in result
     assert "/reasoning" in result
+
+
+@pytest.mark.asyncio
+async def test_btw_dispatches_mid_run():
+    """/btw mid-run must dispatch to its handler, not hit the catch-all.
+
+    /btw spawns a parallel ephemeral side-question task that does NOT
+    interrupt the active conversation (see _handle_btw_command). It's the
+    whole point of the command — asking a side question while the main
+    turn is still working. Before the mid-turn bypass was added, /btw
+    fell through to the "Agent is running — wait or /stop first" catch-all,
+    making it useless in exactly the scenario it was designed for.
+    """
+    runner = _make_runner()
+    runner._handle_btw_command = AsyncMock(
+        return_value='💬 /btw: "what module owns titles?"\nReply will appear here shortly.'
+    )
+
+    result = await runner._handle_message(_make_event("/btw what module owns titles?"))
+
+    runner._handle_btw_command.assert_awaited_once()
+    assert result is not None
+    assert "💬 /btw" in result
+    assert "can't run mid-turn" not in result