Request UK constituency and local-authority impacts separately from the national economy result #4021

What this is about

.py 6.10.0 runs UK national economy reports on a national dataset (microcosm_uk_2024_25) that has no area codes, so a national run no longer returns constituency or local-authority impacts. The simulation API (PolicyEngine/policyengine-sim-api#881) adds a separate run on the local-area dataset that returns only those impacts. The Public API needs a way to ask for that run.

What changes in the Public API

• The annual economy route, GET /{country}/economy/{reform}/over/{baseline}, accepts a new optional query parameter, breakdown=local_areas. The budget-window route does not accept it.
• The parameter is valid only for the UK, with region set to uk or a UK country (country/*), and not with target=cliff. Any other combination, or any other value, returns HTTP 400.
• When the parameter is set, it becomes part of the calculation options, so the request gets its own options hash and its own cached result. Requests without it keep their current options hash and cached results.
• The Public API sends "breakdown": "local_areas" to the simulation API only when the parameter is set, and stores and returns the result the simulation API sends back (kind: "local_areas", with constituency_impact and local_authority_impact).
• If the simulation API refuses a breakdown request with HTTP 400 or 422 (for example, because the deployed .py release cannot run it), the Public API returns a 502 error that says so, instead of a generic 500.
• UK reports for Northern Ireland use country/ni in the Public API's metadata, but .py's region registry names that region country/northern_ireland. The Public API translates the name when it builds the simulation request.
• The .py bundle moves from 6.7.5 to 6.10.0 (this replaces Update PolicyEngine bundle to 6.10.0 #3991), and the reviewed UK default dataset becomes microcosm_uk_2024_25.

Order of deployment

This change has to merge after the simulation API change (PolicyEngine/policyengine-sim-api#881) is deployed with .py 6.10.0, and after the app change that requests the breakdown is merged.