Postcode Configuration Page shows an empty Address Provider when a provider is configured

Describe the issue
OnOpenPage of page 9143 "Postcode Configuration Page W1" (and the GB twin, page 10501 "Postcode Configuration Page") sets the display variable ServiceKeyText only in the record-missing and not-configured branches — both to Disabled. With a correctly configured provider neither branch runs, so the "Address Provider" field renders an empty value (shows as an underscore in the web client) instead of the selected provider. The stored key is already read into PrevValue; it is just never displayed.

Reproduced on BC 28.2 with a configured, working postcode provider. This was previously masked for IdealPostcodes because a provider bug (see the linked IdealPostcodes issue) kept IsConfigured() false, which forced the page into the Disabled branch.

Expected behavior
Reopening the Postcode Configuration Page shows the currently selected provider.

Proposed fix (implemented)
In both pages, replace the last statement of OnOpenPage:

// old
PrevValue := Rec.GetServiceKey();
// new
ServiceKeyText := Rec.GetServiceKey();
PrevValue := ServiceKeyText;

A PR with this change is ready on branch sergey-idealpostcodes:fix/postcode-configuration-page-show-selected-provider and will be opened once this issue is approved.

Start with OnOpenPage on pages 9143 and 10501, "Postcode Configuration Page W1" and "Postcode Configuration Page". Reopen each page with a configured provider and verify that the Address Provider field displays the selected provider rather than an empty value; the proposed fix is already implemented on a contributor branch.