OK Child Care/Child Tax Credit uses federal CDCC potential instead of allowed amount

[PavelMakarchuk]

Summary

The Oklahoma Child Care/Child Tax Credit uses the federal CDCC potential amount instead of the federal CDCC allowed (tax-limited) amount, overstating the credit for filers whose federal tax limits their child care credit.

Originating TAXSIM comparison: PolicyEngine/policyengine-taxsim#964

Legal basis

2025 OK Form 511 packet (Schedule 511-F): the Oklahoma credit is the greater of "20% of the credit for child care expenses allowed by the IRC" or "5% of the child tax credit allowed by the IRC." Schedule 511-F line 1 instructs "Enter your federal child care credit" — i.e., the amount on the federal return (Form 2441 → Schedule 3), which is nonrefundable and limited to tax liability.

Root cause

ok_child_care_child_tax_credit uses tax_unit("cdcc_potential") with the comment "Oklahoma matches the potential credit amount regardless of federal tax liability." That contradicts the form, which uses the allowed credit. (The CTC branch already correctly uses the allowed ok_federal_ctc; this is the same potential-vs-allowed fix applied to the CTC side in #6586, not yet applied to the CDCC side.)

Reproduction (OK HoH, $25,000 wages, $877 interest, 1 child age 5, $3,500 childcare, TY2025)

	CDCC basis	OK credit
PE (current)	potential $870 → 20% = $174	$174
Correct	allowed $225 → 20% = $45; vs 5%×CTC $1,700 = $85	$85

$85 matches the reporter's Form 511 line 15, TaxAct, and the live TAXSIM run. (The local taxsimtest binary only supports years ≤2024, so this is anchored to the form + those three engines rather than a binary re-run.)

Suggested fix

Use the allowed federal CDCC (cdcc) rather than cdcc_potential in the OK CDCC branch.

Integration test

- name: OK child care/child tax credit uses the allowed (not potential) federal CDCC
  period: 2025
  input:
    people:
      head: {age: 45, employment_income: 25_000, taxable_interest_income: 877.45}
      child: {age: 5}
    tax_units: {tax_unit: {members: [head, child], tax_unit_childcare_expenses: 3_500}}
    households: {household: {members: [head, child], state_code: OK}}
  output:
    ok_child_care_child_tax_credit: 85