`blueapi controller run <plan>` hangs after the conclusion of a plan

with config (copied from docs...)
```yaml
env:
sources:
- kind: dodal
module: mx_bluesky.ui_working.devices
- kind: planFunctions
module: mx_bluesky.ui_working.plans

stomp:
host: localhost
port: 61613
auth:
username: guest
# This is for local development only, production systems should use good passwords
password: guest
```
and the `mx-bluesky` branch `i23_and_ui_testing` which has some extremely simple plans of the like

```python
@bpp.run_decorator()
def virtual_relative_move_um(x_um: float, y_um: float) -> MsgGenerator:
"""
Used for click-to-move on an OAV. Moves the gonio a relative distance of x and y to
bring the clicked location to the beam centre. Takes account of the goniometer omega
position in order to do the relative move in the lab frame.
"""
gonio = devices.vgonio()

LOGGER.info(
f"Current position: {yield from bps.rd(gonio.x)}, {yield from bps.rd(gonio.yh)}"
)
yield from bps.rel_set(gonio.x, x_um, wait=True)
yield from bps.rel_set(gonio.yh, y_um, wait=True)
LOGGER.info(
f"New position: {yield from bps.rd(gonio.x)}, {yield from bps.rd(gonio.yh)}"
)
```

where `gonio` is prepared using the old-style `device_instantiation` because I could set the mock arg to `True` there...

running
```bash
╰─>$ blueapi --config stomp.yml controller run virtual_relative_move_um '{"x_um": 4, "y_um": 23}'
2024-11-29 12:21:44,255 - Connecting...
2024-11-29 12:21:44,256 - established connection to host localhost, port 61613
2024-11-29 12:21:44,258 - Subscribing to name='public.worker.event'
```

hangs forever. the plan runs successfully and the worker returns to idle state but the controller (presumably) waits for this event forever.