Fix OH CDCC to use uncapped federal credit for low-income filers

[MaxGhenis]

Summary

• Fixes the OH CDCC formula to correctly apply the federal credit base per ORC § 5747.054(A)
• Adds a low_income_threshold parameter ($20,000) in the OH CDCC parameter directory
• For filers with AGI below $20,000: uses cdcc_potential (federal credit without the IRC § 26 limitation)
• For filers with AGI $20,000–$39,999: now correctly uses cdcc (federal credit limited by IRC § 26 tax liability)
• Expands test coverage to include cases where cdcc < cdcc_potential to verify the bracket-specific behavior

Closes #7682

What was wrong

The previous formula applied cdcc_potential (the uncapped federal credit) to all filers regardless of AGI. This overstated the Ohio credit for middle-income filers ($20,000–$39,999) whose federal credit is reduced by the § 26 tax liability cap.

Statute

ORC § 5747.054(A) specifies:

AGI range	Ohio credit base
Below $20,000	"the credit allowed under section 21 of the Internal Revenue Code without regard to any limitation imposed by section 26" → cdcc_potential
$20,000–$39,999	the federal child and dependent care credit (implicitly subject to § 26) → cdcc
$40,000+	$0

Test plan

• All 7 tests in oh_cdcc.yaml pass (uv run policyengine-core test)
• Tests cover: low-income filer where cdcc < cdcc_potential (confirms cdcc_potential is used), middle-income filer where cdcc < cdcc_potential (confirms cdcc is used), middle-income where both are equal, high-income ineligible, out-of-state ineligible

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (55a0045) to head (13242bd).
⚠️ Report is 37 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #7687      +/-   ##
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+ Coverage   97.82%   100.00%   +2.17%     
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  Files           3         1       -2     
  Lines          92        16      -76     
  Branches        2         0       -2     
===========================================
- Hits           90        16      -74     
+ Misses          1         0       -1     
+ Partials        1         0       -1     

Flag	Coverage Δ
unittests	100.00% <100.00%> (+2.17%)	⬆️

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[MaxGhenis]

Form worksheet verification

Verified against the 2024 OH IT 1040 Booklet, page 45 — "Child and Dependent Care Credit Worksheet (Ohio Schedule of Credits, Line 6)":

• Line 2: Form 2441, line 9c = cdcc_potential (credit before IRC §26 cap)
• Line 3: 25% of Form 2441, line 11 = 25% × cdcc (credit AFTER IRC §26 cap)
• Line 4: If line 1 < $20,000 → enter Line 2. If $20,000 ≤ line 1 < $40,000 → enter Line 3.

For the integration test case (AGI ~$39,645):

• cdcc_potential = $660, cdcc = $0 (federal tax liability is $0)
• Line 3 = 25% × $0 = $0
• Updated expected oh_cdcc from 165 → 0 and oh_income_tax from 273 → 414

Source: https://dam.assets.ohio.gov/image/upload/v1735920104/tax.ohio.gov/forms/ohio_individual/individual/2024/it1040-booklet.pdf