add regression test for OpenOptionsExt downstream compat Following up on rust-lang/rust#153491, which added a warning comment there, but no automated guardrail to prevent another breaking change like rust-lang/rust#149718 This adds a simple windows-only ui test that manually implements `OpenOptionsExt`. That way, if someone accidentally adds another required method to the trait, we catch it before it reaches stable and breaks downstream crates like Tokio again. Closes rust-lang/rust#153486