Partial roll back VFS refactor due internal breakages

PiperOrigin-RevId: 862210191
Change-Id: Ia15a345a267bcef4b593d507d54f1668da0c0611

test/xml/xml_api_test.cc

@@ -126,7 +126,7 @@ TEST_F(LoadXmlTest, InvalidFileFails) {
   std::array<char, 1000> error;
   mjSpec* spec = mj_parseXML("invalid", nullptr, error.data(), error.size());
   EXPECT_THAT(spec, IsNull()) << "Expected model loading to fail.";
-  EXPECT_THAT(error.data(), HasSubstr("Error opening file"));
+  EXPECT_THAT(error.data(), HasSubstr("No such file or directory"));
 }
 
 TEST_F(MujocoTest, SaveXmlShortString) {