Fix C++ formatting for multi-line string in error message

Co-authored-by: jtramm <1009059+jtramm@users.noreply.github.com>

Author: Copilot

src/random_ray/flat_source_domain.cpp

@@ -1095,7 +1095,8 @@ void FlatSourceDomain::convert_external_sources()
         auto it = point_source_map_.find(key);
         if (it != point_source_map_.end()) {
           fatal_error(
-            "Multiple point sources detected in the same subdivided source region."
+            "Multiple point sources detected in the same subdivided source "
+            "region."
             "This is not currently supported in the random ray solver.");
         }
         point_source_map_[key] = es;