test(constraint): pin the three Pack-C review hazards as regression tests

Per review: (H1) exactly-two-category binding already has its meaningful-fail twin; (H2) a weighted total whose unit does not match the coefficient unit refuses missing_weighted_total (never a cross-unit sum); (H3) answer-choice ties EXACTLY or refuses no_matching_option (never nearest). All three were already handled by construction; these pin them so they stay handled.

tests/test_answer_choices.py

@@ -68,6 +68,13 @@ def test_no_matching_option_refuses() -> None:
     assert isinstance(out, Refusal) and out.reason == "no_matching_option"
 
 
+def test_exact_match_only_never_nearest() -> None:
+    # Hazard: a proven value with no exact option REFUSES — it never snaps to the nearest
+    # (10 or 12). Exact-or-refuse is the wrong=0 boundary for answer-choice tie-in.
+    out = verify_answer_choice(11, {"A": 10, "B": 12, "C": 13, "D": 14}, "B")
+    assert isinstance(out, Refusal) and out.reason == "no_matching_option"
+
+
 def test_ambiguous_duplicate_options_refuse() -> None:
     out = verify_answer_choice(4, {"A": 4, "B": 4}, None)
     assert isinstance(out, Refusal) and out.reason == "ambiguous_options"

tests/test_constraint_reader.py

@@ -76,6 +76,17 @@ def test_coefficient_unit_mismatch_refuses() -> None:
     assert isinstance(out, Refusal) and out.reason == "coefficient_unit_mismatch"
 
 
+def test_weighted_total_in_wrong_unit_refuses() -> None:
+    # Hazard: the weighted total's unit must match the coefficient unit. Coefficients are in
+    # students; a total in DOLLARS matches no coefficient unit, so the weighted equation is
+    # never assembled -> missing_weighted_total. The reader never sums across units.
+    out = read_constraint_problem(
+        "A school rents 6 buses. Each large bus holds 50 students and each small bus holds "
+        "30 students. The buses cost 260 dollars in total. How many large buses are there?"
+    )
+    assert isinstance(out, Refusal) and out.reason == "missing_weighted_total"
+
+
 def test_off_category_query_refuses() -> None:
     out = read_constraint_problem(
         "A school rents 6 buses for a trip. Each large bus holds 50 students and each small bus "