i15-1: Make sure the centring plan can run in mock mode

To test the full stack it would be useful if the `centre_plan` runs happily with all devices in mock.

Currently it errors with:
```
bluesky.utils.FailedStatus: .wait_with_error_message at 0x7fad8d630440>, errored: ValueError('0 is not a valid SoftInState')>
```

## Acceptance Criteria
- The `centre_plan` can be run with devices in mock
- There is a test for this

Start by running the `centre_plan` with all devices in mock mode and reproduce the `SoftInState` ValueError shown in the issue. Trace the mock-device path for `zebra-inputs-soft_in_1`, then add a test covering the plan; done means the plan runs successfully with mocked devices and the test passes.

Written by the indexing model from the issue text.