fix: Include <string> directly in LoanBroker_test.cpp

The new overflow test calls std::to_string but only relied on a transitive
include for it, which clang-tidy's include-cleaner check flagged in CI.

src/test/app/lending/LoanBroker_test.cpp

@@ -59,6 +59,7 @@
 #include <functional>
 #include <memory>
 #include <optional>
+#include <string>
 #include <string_view>
 #include <tuple>
 #include <utility>