Undulator sets completing successfully, gap not changed

During a weekend run, the undulator was requested to move, the gap was set, but the RBV did not change, and the set did not timeout.

https://graylog.diamond.ac.uk/messages/graylog-daq-mx-hyperion_22/7b57eb42-a0f0-11f0-8f22-6e2c9d89af67

See slack conversation at
https://diamondlightsource.slack.com/archives/C048RD0KFFF/p1759742834238719

Several samples were collected with incorrect undulator gap because the RBV isn't validated.

This was on mx-bluesky 1.5.0 which doesn't have the latest `ophyd-async` with limit validation.

Suspect that undulator was reporting that move was done however DMOV has no logs.

Suggest additional check in the set implementation to read back the RBV and raise exception if outside tolerance.

## Acceptance Criteria
- Specific criteria that will be used to judge if the issue is fixed