Unable to onboard a Remote Device if MQTT broker is not present

### Current Behavior

When running the `FlowFuse Device Agent` on an instance where the MQTT broker is disabled, the agent attempts to fall back to HTTP polling. However, the onboarding process fails with continuous `409 Conflict` errors on the `/api/v1/devices/$device_id/live/state` endpoint.

Initial investigation: https://flowfuse.slack.com/archives/C032Q63FGG1/p1770384564803619

### Expected Behavior

A Remote Instance onboarding should complete successfully.

### Steps To Reproduce

1. Deploy FlowFuse Platform **without** a MQTT broker.
2. Install FlowFuse Device Agent (`3.7.2`) on a remote device.
3. Register and bind the device to an Application in the platform.
4. Observe the device agent logs and platform ingress logs.

### Environment

- FlowFuse version: `2.26.2`
- Node.js version:
- npm version:
- Platform/OS: AWS EKS cluster (Kubernetes)
- Browser:

### Have you provided an initial effort estimate for this issue?

I have provided an initial effort estimate