fix: revert config-inversion changes, no longer needed with "" default

Since _DD_ROOT_CPP_SESSION_ID now defaults to "" (not nullptr), the
config-inversion tool doesn't need the nullptr specialization. Revert
to original to_string_any and fix implementation marker to "A".

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: khanayan123

supported-configurations.json

@@ -283,7 +283,7 @@
     "_DD_ROOT_CPP_SESSION_ID": [
       {
         "default": "",
-        "implementation": "B",
+        "implementation": "A",
         "type": "STRING"
       }
     ]

tools/config-inversion/main.cpp

@@ -11,17 +11,12 @@ namespace fs = std::filesystem;
 namespace env = datadog::tracing::environment;
 
 template <typename T>
-nlohmann::json to_json_default(const T& value) {
+std::string to_string_any(const T& value) {
   std::ostringstream oss;
   oss << value;
   return oss.str();
 }
 
-template <>
-nlohmann::json to_json_default(std::nullptr_t const&) {
-  return nullptr;
-}
-
 nlohmann::json build_configuration() {
   nlohmann::json j;
   j["version"] = "2";
@@ -36,7 +31,7 @@ nlohmann::json build_configuration() {
 #define X(NAME, TYPE, DEFAULT_VALUE)                                       \
   do {                                                                     \
     auto obj = nlohmann::json::object();                                   \
-    obj["default"] = to_json_default(DEFAULT_VALUE);                       \
+    obj["default"] = to_string_any(DEFAULT_VALUE);                         \
     obj["implementation"] = "A";                                           \
     obj["type"] = QUOTED(TYPE);                                            \
     supported_configurations[QUOTED(NAME)] = nlohmann::json::array({obj}); \