b294d15558
This change introduces new functions to load an `mjModel` from an XML string. Two overloads are provided: one that creates a temporary VFS and one that uses a MjVFS arg. Also updated other places where `from_xml_string` should be used: demos, readme, etc. PiperOrigin-RevId: 882581762 Change-Id: I8428196f8f56ae9630fcd4f0bb6763cdc1c43da0