fix(telemetry): link in-memory exporter component into xrpl_tests

The umbrella opentelemetry-cpp target's link set omits the in-memory
exporter library, so xrpl_tests failed to link with an undefined
reference to InMemorySpanExporterFactory::Create. The SpanGuardScope
tests use an InMemorySpanExporter to read exported spans. Link the
opentelemetry-cpp::exporter_in_memory component explicitly. Test-only:
production code never uses this exporter.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/tests/libxrpl/CMakeLists.txt

@@ -67,9 +67,15 @@ endif()
 # The telemetry test module exercises SpanGuard/Telemetry, which link against
 # the OpenTelemetry SDK when the telemetry build option is enabled.
 if(telemetry)
+    # The umbrella opentelemetry-cpp target does NOT include the in-memory
+    # exporter in its link set, so link that component explicitly: the
+    # SpanGuardScope tests drive an InMemorySpanExporter to read exported
+    # spans. Test-only -- production code never uses this exporter.
     target_link_libraries(
         xrpl_tests
-        PRIVATE opentelemetry-cpp::opentelemetry-cpp
+        PRIVATE
+            opentelemetry-cpp::opentelemetry-cpp
+            opentelemetry-cpp::exporter_in_memory
     )
 endif()