feat: support multiple households in calculation endpoints

[nikhilwoodruff]

Summary

Changed entity inputs from single dicts to lists of dicts, enabling callers to submit multiple households with entity relational dataframes. The caller specifies {entity}_id fields in each entity and person_{entity}_id fields in people to link them together.

Changes

• Updated HouseholdCalculateRequest and HouseholdImpactRequest schemas: entity fields (benunit, marital_unit, family, spm_unit, tax_unit, household) changed from dict[str, Any] to list[dict[str, Any]]
• Updated HouseholdCalculateResponse to return household as a list
• Rewrote local calculation functions (_calculate_household_uk, _calculate_household_us) to build entity dataframes directly with proper ID relationships
• Updated Modal functions (simulate_household_uk, simulate_household_us) with the same logic
• Fixed imports for MicroDataFrame and UKYearData/USYearData
• Added multi-household tests for both UK and US

Example

{
  "tax_benefit_model_name": "policyengine_us",
  "people": [
    {"person_id": 0, "person_household_id": 0, "person_tax_unit_id": 0, "age": 40, "employment_income": 70000},
    {"person_id": 1, "person_household_id": 1, "person_tax_unit_id": 1, "age": 30, "employment_income": 50000}
  ],
  "tax_unit": [
    {"tax_unit_id": 0, "state_code": "CA"},
    {"tax_unit_id": 1, "state_code": "NY"}
  ],
  "household": [
    {"household_id": 0, "state_fips": 6},
    {"household_id": 1, "state_fips": 36}
  ],
  "year": 2024
}

Testing

All 10 household tests pass, including 2 new multi-household tests.