Fix memory and resource leaks in failing resource provider open callbacks

When an mjpResourceProvider's open callback returns 0 (failure), MuJoCo does
not invoke the corresponding close callback. Previously, several resource
provider implementations allocated heap memory or system file handles before
encountering an error, failing to clean them up before returning 0. This change addresses these leaks across the codebase and clarifies the API contract.

PiperOrigin-RevId: 929803942
Change-Id: Ib7344033726895cdf037a8d9356ffc183c78c2d3

include/mujoco/mjplugin.h

@@ -33,7 +33,10 @@ struct mjResource_ {
 };
 typedef struct mjResource_ mjResource;
 
-// callback for opening a resource, returns zero on failure
+// callback for opening a resource, returns zero on failure.
+// Note: If opening fails, the close callback will not be called. Therefore, the
+// open callback is responsible for cleaning up any allocated memory before
+// returning 0.
 typedef int (*mjfOpenResource)(mjResource* resource);
 
 // callback for reading a resource