Postcode Configuration Page: display the stored provider selection on open

[sergey-idealpostcodes]

Fixes #11255

OnOpenPage of page 9143 "Postcode Configuration Page W1" and page 10501 "Postcode Configuration Page" (GB) sets the ServiceKeyText display variable only in the record-missing / not-configured branches, so with a configured provider the Address Provider field renders empty (an underscore in the web client). The stored key was already read into PrevValue — it was just never displayed.

One-line fix in each page: load the stored service key into ServiceKeyText and keep PrevValue for cancel-restore. Together with #11254, the field then reads IdealPostcodes for that provider.

🤖 Generated with Claude Code

[github-actions]

Issue #11255 is not valid. Please make sure you link an issue that exists, is open and is approved.

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