Normalise country/ni and country/northern_ireland before the economy cache lookup #4037

Problem

A UK request for Northern Ireland can arrive as country/ni (the UK metadata name, which the app sends) or as country/northern_ireland (the name in policyengine.py's UK region registry). The API maps the name only when it builds the simulation API payload, in EconomyService._setup_region (policyengine_api/services/economy_service.py, via UK_REGION_REGISTRY_CODES in constants.py). _with_normalized_region normalises US regions only, so the economy cache key is built from the region as requested.

As a result, the same Northern Ireland run (national or breakdown=local_areas) gets two cache entries and can be computed twice, once for each name. This came up in the review of #4024 (point 2).

Impact

Small today: the app always sends country/ni. It matters for any client that uses the registry name, and it doubles the cost of a Northern Ireland run when both names are used.

Proposed fix

Normalise UK regions before the cache lookup, in the same place as the US normalisation: map country/northern_ireland to one canonical name (or map both to the registry name), so both names share one cache entry. Keep saved records and existing URLs working. Add a test that both names resolve to the same cache key.

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