Fix RCC other-dependent credit semantics

[daphnehanse11]

Summary

• Change the refundable credit conversion per-other-dependent credit to count all tax-unit dependents who are not CTC-qualifying children.
• Update parameter descriptions so they no longer describe the credit as limited to EITC-qualifying children.
• Add regression coverage for an adult dependent and a full tax-unit-member coverage scenario.

Fixes #8552.

Tests

• uv run python -c 'import os; from policyengine_core.scripts import build_tax_benefit_system; from policyengine_core.tools.test_runner import run_tests; tbs = build_tax_benefit_system("policyengine_us", [], []); status = run_tests(tbs, ["policyengine_us/tests/policy/contrib/refundable_credit_conversion/dependent_definition.yaml"]); print(f"run_tests_status={status}", flush=True); os._exit(status)'
  • Collected 9 cases and printed 9 passing dots, then stalled before pytest printed its final summary; I killed the stuck process.