Segment change requests silently discard custom field edits

## Summary

Custom fields (metadata) on a segment can be edited while segment change requests are enabled, and the edit is carried into the change request, but it is never shown to the approver and never applied when the change request is committed. The edit is silently lost.

## Repro

1. Enable Segment Change Requests on a project, and configure a custom field for segments.
2. Edit a segment, change a value on the Custom Fields tab, and save. A change request is created, as expected.
3. Open the change request. The diff shows Name, Description and Rules only — the custom field change is not shown anywhere.
4. Approve and publish the change request.
5. The live segment's custom field is unchanged.

## Expected

The custom field edit is published with the rest of the change request, shown in the change request diff, and is correctly applied OR metadata is excluded from the change requests entirely.