Fix clang-tidy misc-const-correctness in heterogeneous sentinel test

json_str is only read via .data()/.size() and never reassigned, so
clang-tidy correctly flags it as const-able. Verified against the exact
CI job (silkeh/clang:dev, ci_clang_tidy target) by running clang-tidy
directly on this file plus the five binary-format sentinel tests
touched by prior commits; all are now clean.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-user_defined_input.cpp

@@ -184,7 +184,7 @@ struct CustomSentinel
 
 TEST_CASE("Parse with heterogeneous iterator and sentinel types")
 {
-    std::string json_str = R"({"key":"value"})";
+    const std::string json_str = R"({"key":"value"})";
     const char* end_ptr = json_str.data() + json_str.size();
 
     // Parse using pointer and sentinel (different types)