feat(sp41): 120-day timely-filing gate with per-batch override

backend/tests/test_rebill_timely_filing.py

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+"""120-day DOS-age gate. CO Medicaid's timely-filing window is 120 days from DOS."""
+from datetime import date
+from decimal import Decimal
+from cyclone.rebill.timely_filing import timely_filing_decision, FILING_WINDOW_DAYS
+
+
+def test_visit_within_window_is_rebillable():
+    decision = timely_filing_decision(
+        dos=date(2026, 6, 1), as_of=date(2026, 7, 7), override=False
+    )
+    assert decision.rebillable is True
+
+
+def test_visit_past_window_excluded_by_default():
+    decision = timely_filing_decision(
+        dos=date(2026, 1, 1), as_of=date(2026, 7, 7), override=False
+    )
+    assert decision.rebillable is False
+    assert decision.days_old == 187
+
+
+def test_visit_at_exact_120_days_inclusive():
+    decision = timely_filing_decision(
+        dos=date(2026, 3, 9), as_of=date(2026, 7, 7), override=False
+    )
+    # 2026-03-09 → 2026-07-07 = 120 days
+    assert decision.days_old == 120
+    assert decision.rebillable is True
+
+
+def test_override_flag_relaxes_the_gate():
+    decision = timely_filing_decision(
+        dos=date(2026, 1, 1), as_of=date(2026, 7, 7), override=True
+    )
+    assert decision.rebillable is True
+
+
+def test_window_constant_is_120():
+    assert FILING_WINDOW_DAYS == 120