Expose product length calculation through MCP

tests/test_roll_calculation.py

@@ -9,6 +9,7 @@ from roll_calculation import (
     _canonical_name,
     article_production_site,
     calculate_material_weight,
+    calculate_product_length,
     calculate_roll,
     get_article,
     modify_calculation,
@@ -212,6 +213,37 @@ class RollCalculationTests(unittest.TestCase):
             item["field"] for item in result["invalid"]
         ])
 
+    def test_product_length_characterizes_article_218500_with_ranges_and_weight(self):
+        result = calculate_product_length(
+            target_roll_diameter_mm=600,
+            core_diameter_mm=140,
+            thickness_mm=4.311611,
+            thickness_stddev_mm=0.249999,
+            width_m=6.0,
+            area_weight_g_m2=495.395833,
+        )
+
+        self.assertEqual(result["status"], "success")
+        calculation = result["calculation"]
+        self.assertAlmostEqual(calculation["minimum_product_length_m"], 55.5634372661)
+        self.assertAlmostEqual(calculation["average_product_length_m"], 62.0068774341)
+        self.assertAlmostEqual(calculation["maximum_product_length_m"], 70.1407868061)
+        self.assertAlmostEqual(
+            calculation["material_weights_kg"]["average_kg"], 184.3076921892
+        )
+
+    def test_product_length_rejects_target_diameter_not_larger_than_core(self):
+        result = calculate_product_length(
+            target_roll_diameter_mm=140,
+            core_diameter_mm=140,
+            thickness_mm=4,
+        )
+
+        self.assertEqual(result["status"], "invalid_parameter")
+        self.assertEqual(
+            result["invalid"][0]["field"], "target_roll_diameter_mm"
+        )
+
     def test_diameter_only_calculation_does_not_require_width(self):
         result = self.calculate_fixture(
             {