Update Texas CCS parameter values 2026

[hua7450]

Fixes #7336
Fixes #6685

Summary

Updates the Texas CCS copayment (Parent Share of Cost) formula and parameters to match the official TWC PSoC Calculator.

How this fixes #6685

Issue #6685 reported that the BCY2025 PSoC chart's "Add 0.15% for each additional child" text doesn't match the per-family-size tables below it. The "0.15%" is misleading — it's actually the rate increment per 10% SMI bracket step, not a flat rate.

By extracting the formulas from the official TWC PSoC Calculator Excel files (BCY2025, BCY2026), we found the actual calculation:

• First child rate: base_rate + slope × (SMI% - min_SMI%), capped at 7%
• Additional child rate: SMI% × 1.5% per child (not a flat 0.15%)
• Total: capped at 7% of income

The additional child rate varies by SMI bracket (e.g., 0.30% at 20% SMI, 0.75% at 50% SMI), which matches the per-family-size tables in the chart. Both BCY2025 and BCY2026 use the same formula with the same constants.

Changes

Parameters

• first_child.yaml: Stores two PSoC chart endpoints (1% SMI → 2.00%, 75% SMI → 6.93%); formula derives slope at runtime
• additional_child.yaml: Stores the factor (0.015) directly; rate = SMI% × 0.015 per additional child
• maximum.yaml: Unchanged (7% cap from 40 TAC §809.19)

Formula (tx_ccs_copay.py)

• Replaced bracket-based np.interp() interpolation with exact linear equations derived from chart endpoint parameters
• First child rate: linear function derived from two stored bracket points
• Additional child rate: smi_ratio × factor × num_additional_children
• Total rate capped at 7%, then applied to income

Tests

• Added test cases validated against official TWC PSoC Estimation Calculator
• Updated error margin to 0.5 (official calculator rounds to whole dollars)

Regulatory Authority

• 40 TAC §809.19 - Parent Share of Cost (authorizes sliding fee scale, caps at 7%)
• TWC BCY2025 PSoC Sliding Fee Scale
• TWC BCY2026 PSoC Sliding Fee Scale

[codecov]

Codecov Report

❌ Patch coverage is 78.94737% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.77%. Comparing base (a46b5a8) to head (818e756).
⚠️ Report is 33 commits behind head on main.

Files with missing lines	Patch %	Lines
...v/states/tx/twc/ccs/payment/tx_ccs_payment_rate.py	54.54%	4 Missing and 1 partial ⚠️
...gov/states/tx/twc/ccs/tx_ccs_child_age_category.py	76.92%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7337      +/-   ##
==========================================
+ Coverage   86.56%   89.77%   +3.20%     
==========================================
  Files          12        3       -9     
  Lines         134       88      -46     
  Branches        0        2       +2     
==========================================
- Hits          116       79      -37     
+ Misses         18        7      -11     
- Partials        0        2       +2     

Flag	Coverage Δ
unittests	89.77% <78.94%> (+3.20%)	⬆️

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