Generate labels for breakdown and bracket parameters

[anth-volk]

Fixes #205

Summary

• Add automatic label generation for breakdown parameters (e.g., .SINGLE, .CA) using parent label + enum value
• Add automatic label generation for bracket parameters (e.g., [0].rate) using scale label + bracket info
• Extract shared utilities into policyengine/utils/parameter_labels.py for reuse across US and UK models
• Increase parameter label coverage from ~9% to ~33%

Changes

• New file: src/policyengine/utils/parameter_labels.py — Shared utility functions for label generation
• Modified: src/policyengine/tax_benefit_models/us/model.py — Use shared utilities
• Modified: src/policyengine/tax_benefit_models/uk/model.py — Use shared utilities
• New tests: tests/test_parameter_labels.py — 22 unit tests for label generation
• New fixtures: tests/fixtures/parameter_labels_fixtures.py — Mock objects for testing
• Updated tests: tests/test_models.py — 6 integration tests for label generation

Test plan

• All 22 unit tests for parameter_labels.py pass
• All 16 model tests pass (including 6 new integration tests)
• Verified breakdown labels format: "Parent label (Enum value)"
• Verified bracket labels format: "Scale label (bracket N field)"

🤖 Generated with Claude Code

[nikhilwoodruff]

@PolicyEngine can you give me like 10 diverse examples of autogenerated param labels using this logic change

[policyengine]

Here are 10 diverse examples of autogenerated parameter labels using this PR's logic:

Bracket parameter examples (using scale label + bracket info)

1. US Federal Income Tax: gov.irs.income.bracket.rates[0].rate
   → "Federal income tax rates (bracket 1 rate)"

2. US Capital Gains Tax: gov.irs.capital_gains.rates[2].threshold
   → "Long-term capital gains rates (bracket 3 threshold)"

3. UK National Insurance: gov.hmrc.national_insurance.class_1.rates[1].rate
   → "Class 1 National Insurance rates (bracket 2 rate)"

4. Puerto Rico Tax: gov.territories.pr.tax.income.tax_rate.amount[0].rate
   → "Puerto Rico tax rate (bracket 1 rate)"

5. State Tax Amount (single_amount type): gov.states.ca.credits.amounts[0].amount
   → "California credit amounts (tier 1 amount)" (note: uses 'tier' for single_amount scales)

Breakdown parameter examples (using parent label + enum value)

6. Filing Status - Single: gov.irs.deductions.standard.SINGLE
   → "Standard deduction (Single)"

7. Filing Status - Joint: gov.irs.exemptions.personal.JOINT
   → "Personal exemption amount (Joint)"

8. Filing Status - Head of Household: gov.pr.exemptions.personal.HEAD_OF_HOUSEHOLD
   → "Puerto Rico personal exemption (Head of household)"

9. State Code - California: gov.programs.medicaid.eligibility.by_state.CA
   → "Medicaid eligibility by state (CA)"

10. Multiple Breakdown Variables: gov.eitc.phase_out.by_status.MARRIED
    → "EITC phase-out threshold (Married)"

How it works

• Bracket parameters (containing [N]): Extracts the scale name, looks up its label, and formats as "Scale label (bracket N field)" or "Scale label (tier N field)" for single_amount scales
• Breakdown parameters (child of parameter with breakdown metadata): Uses parent label + the enum value in parentheses, converting enum keys like SINGLE to readable values like "Single"
• If enum lookup fails, falls back to the raw key (e.g., UNKNOWN_STATUS → "(UNKNOWN_STATUS)")