Fix IndexError in variable bound propagators during unsolvable constraint solving

Co-authored-by: mballance <1340805+mballance@users.noreply.github.com>

Author: Copilot

src/vsc/model/variable_bound_max_propagator.py

@@ -34,7 +34,8 @@ def propagate(self):
         must_propagate = False
         if i >= 0:
 #            print("i: " + str(i) + " " + str(self.target.domain.range_l[i][0]))
-            if range_l[i][1] > max_v:
+            # Check if range_l is not empty before accessing
+            if len(range_l) > i and range_l[i][1] > max_v:
                 range_l[i][1] = max_v
                 must_propagate = True
 
@@ -43,6 +44,7 @@ def propagate(self):
                 must_propagate = True
 #                print("Removing domain element " + str(range_l[i+1]))
                 self.target.domain.range_l = range_l[:i+1]
+                range_l = self.target.domain.range_l  # Update local reference
         else:
 #            print("ran off the end")
             pass

src/vsc/model/variable_bound_min_propagator.py

@@ -37,8 +37,10 @@ def propagate(self):
                 # Need to trim off full range elements
                 must_propagate = True
                 self.target.domain.range_l = range_l[i:]
+                range_l = self.target.domain.range_l  # Update local reference
 
-            if min_v > range_l[0][0]:
+            # Check if range_l is not empty before accessing
+            if len(range_l) > 0 and min_v > range_l[0][0]:
                 range_l[0][0] = min_v
                 must_propagate = True
         else: