Fill the withheld-area code lists in UK local-area breakdown results #988

Part of #968.

Problem

#976 made the contract and the gateway accept constituency_withheld_codes and local_authority_withheld_codes on a UK local-area breakdown result (kind: local_areas), and it reached production on 3 Nov. No executor fills them yet, so a breakdown result still leaves out each withheld constituency or local authority without saying which. Clients can't tell an area disclosure control withheld from one the run never estimated.

GitHub closed #968 when #976 merged. This issue tracks the remaining executor change, #975.

What should happen

The executor fills both lists on every breakdown=local_areas result. Each holds, sorted, the ONS codes of that level's areas the run estimated (the areas with household weight) but disclosure control didn't publish:

• [] when nothing at that level was withheld;
• every estimated area when withholding can't resolve the disclosure findings and nothing is published;
• never an area the run didn't estimate, such as one with no households in the dataset.

Done when

• A local-area breakdown result carries both lists, sorted, with no code of a published area.
• A breakdown without the disclosure-controlled output fails rather than returning without the lists.
• The beta local-area breakdown check requires both lists.