Refactor user codebase to use LoadResource helper method.

* Fix small bug when registering resource providers.
* Add test cases for OS filesystem fallback with VFS.
* Rewrite LoadModelFromString using a resource provider (now can pass custom VFSs).

PiperOrigin-RevId: 526959705
Change-Id: Iee1773e5584535d868aab6b206d1bf362e600c8a

src/engine/engine_plugin.cc

@@ -588,7 +588,7 @@ int mjp_registerResourceProviderInternal(const mjpResourceProvider* provider) {
         break;
       }
 
-      if (!vfs_provider) {
+      if (!vfs_provider && existing.prefix != nullptr) {
         int n = std::strlen(provider->prefix);
         int m = std::strlen(existing.prefix);