fix: enforce thermal boundary dof endpoints

include/fesa/elements/ds4_thermal.h

@@ -2,6 +2,7 @@
 #define FESA_ELEMENTS_DS4_THERMAL_H_
 
 #include <array>
+#include <limits>
 #include <utility>
 #include <vector>
 
@@ -14,6 +15,9 @@ namespace fesa {
 /// state.
 class Ds4ThermalDefinition final : public ElementDefinition {
  public:
+  /// @brief Marks that DS4 has no entry in Domain::Properties().
+  static constexpr EntityIndex kNoPropertyIndex =
+      (std::numeric_limits<EntityIndex>::max)();
   Ds4ThermalDefinition(SourceEntityId source_id,
                        std::array<EntityIndex, 4> node_indices,
                        EntityIndex section_index, SourceLocation location);
@@ -74,7 +78,7 @@ inline EntityIndex Ds4ThermalDefinition::MaterialIndex() const noexcept {
 }
 
 inline EntityIndex Ds4ThermalDefinition::PropertyIndex() const noexcept {
-  return section_index_;
+  return kNoPropertyIndex;
 }
 
 inline EntityIndex Ds4ThermalDefinition::ThermalSectionIndex() const noexcept {