test: add root_session_id singleton test

Verify get_or_init returns the first runtime ID on subsequent calls
with different values (first-writer-wins semantics).

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

Author: khanayan123

test/CMakeLists.txt

@@ -18,6 +18,7 @@ add_executable(tests
 
     # telemetry test cases
     telemetry/test_configuration.cpp
+    telemetry/test_root_session_id.cpp
     telemetry/test_telemetry.cpp
 
     # test cases

test/telemetry/test_root_session_id.cpp

@@ -0,0 +1,21 @@
+#include <datadog/runtime_id.h>
+
+#include "../test.h"
+#include "root_session_id.h"
+
+using namespace datadog::tracing;
+
+#define ROOT_SESSION_ID_TEST(x) \
+  TEST_CASE(x, "[telemetry],[telemetry.root_session_id]")
+
+ROOT_SESSION_ID_TEST("get_or_init returns the first runtime ID") {
+  auto first_rid = RuntimeID::generate();
+  auto second_rid = RuntimeID::generate();
+
+  const auto& result1 = root_session_id::get_or_init(first_rid.string());
+  const auto& result2 = root_session_id::get_or_init(second_rid.string());
+
+  CHECK(result1 == first_rid.string());
+  CHECK(result2 == first_rid.string());
+  CHECK(result1 == result2);
+}