fix(tests): add getMetricsRegistry() override to TestServiceRegistry

ServiceRegistry gained the pure virtual getMetricsRegistry() in phase 7
but TestServiceRegistry was never updated. Returns nullptr since tests
don't need a real MetricsRegistry.

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

src/tests/libxrpl/helpers/TestServiceRegistry.h

@@ -334,6 +334,12 @@ public:
         throw std::logic_error("TestServiceRegistry::getTelemetry() not implemented");
     }
 
+    telemetry::MetricsRegistry*
+    getMetricsRegistry() override
+    {
+        return nullptr;
+    }
+
     // Configuration and state
     bool
     isStopping() const override