test(ml): add threshold monotonicity coverage for is_flagged

Author: AAdewunmi

tests/test_ml_threshold_monotonicity.py

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+# path: policylens/tests/test_ml_threshold_monotonicity.py
+"""
+Tests for threshold monotonicity.
+"""
+
+from __future__ import annotations
+
+from policylens.apps.claims.ml.thresholds import is_flagged
+
+
+def test_threshold_monotonicity_holds_for_increasing_scores():
+    """If score increases, flagged status must not flip from True to False."""
+    threshold = 0.6
+    scores = [0.0, 0.2, 0.59, 0.6, 0.61, 0.9, 1.0]
+
+    flagged = [is_flagged(score=s, threshold=threshold) for s in scores]
+
+    # Once flagged becomes True, it must stay True for higher scores.
+    seen_true = False
+    for f in flagged:
+        if f:
+            seen_true = True
+        if seen_true:
+            assert f is True