Standardize frontend on country_id instead of tax_benefit_model_name

[anth-volk]

Problem

The frontend uses multiple conventions for identifying which country model to use when calling API v2 endpoints:

• Some calls send country_id ("us"/"uk")
• Some send tax_benefit_model_name with underscores ("policyengine_us")
• Some send tax_benefit_model_name with hyphens ("policyengine-us")
• The core Household type uses tax_benefit_model_name: TaxBenefitModelName with underscore values
• Conversion functions (countryIdToModelName, modelNameToCountryId) exist to bridge formats
• COUNTRY_TO_MODEL_NAME map stored wrong values (underscores instead of hyphens)

This causes:

• Broken metadata fetching (frontend sends underscores, DB stores hyphens)
• Infinite loops when model lookup fails and retries
• Unnecessary complexity with format conversion utilities

Solution

1. Change Household type field from tax_benefit_model_name: TaxBenefitModelName to country_id: CountryId
2. Update all API modules (datasets, regions, parameters, variables, economyAnalysis, simulations, households) to send country_id
3. Update all adapters, hooks, migration orchestrators, and utilities
4. Deprecate countryIdToModelName/modelNameToCountryId as identity functions
5. Deprecate TaxBenefitModelName type as alias for CountryId
6. Update all test fixtures and unit tests

Supersedes #774. Companion to PolicyEngine/policyengine-api-v2-alpha#108.