fix(tests): use the renamed node_store namespace in the registry mock

The nodestore namespace became xrpl::node_store when develop was merged
in, but one mock override still named the old spelling, so it did not
match the ServiceRegistry signature it overrides. The sibling mock in
TestServiceRegistry.h was already correct.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/tests/libxrpl/telemetry/MetricsRegistry.cpp

@@ -557,7 +557,7 @@ public:
     {
         throwUnimplemented();
     }
-    NodeStore::Database&
+    node_store::Database&
     getNodeStore() override
     {
         throwUnimplemented();