Restore retract_to(max_index()) test in retract.cpp with corrected assertion

Restore the deleted test lines that verify retract_to(max_index()) is a
no-op. The original code had a buggy assertion (comparing max_index()
against loop index i, which could diverge due to prior retractions) and
a missing throw keyword. Fix both: save max_index before the call and
verify it's unchanged after.

Co-authored-by: achamayou <4016369+achamayou@users.noreply.github.com>

Author: Copilot

test/retract.cpp

@@ -51,6 +51,12 @@ int main()
           mt.retract_to(mt.max_index() - 1);
           total_retractions++;
           mt.insert(hashes[i]);
+          const auto max_before = mt.max_index();
+          mt.retract_to(mt.max_index());
+          if (mt.max_index() != max_before)
+          {
+            throw std::runtime_error("retract_to(max_index()) changed max_index");
+          }
         }
 
         if ((std::rand() / (double)RAND_MAX) > 0.95)