Federal law has had little impact on the resolution of futility disputes. There have been notable exceptions like Baby K and EMTALA. But in general, federal statutes and regulations are not nearly as relevant as state law.
Most states have some statutory provisions that (purport to) permit healthcare providers to refuse to comply with instructions or decisions for treatment that are contrary to the provider's professional judgment and/or the professional's conscience. Most are ineffective, because they condition immunity on compliance with the standard of care. Texas law includes no such condition.
Some states (like Texas) expressly permit cliicians to stop life sustaining treatment without consent. Other states (like Idaho) expressly prohibit it. But most states have no clear green or red light.