uv

242b3981 - Prepare Rust for test automations (#20557)

Prepare Rust for test automations (#20557) The network-disabled rebase sandbox cannot install the pinned Rust toolchain, fetch missing dependencies, or provide `cargo-nextest` after it checks out a pull request; [a failed rebase job](https://github.com/astral-sh/uv-dev/actions/runs/29608044060/job/87976060317) shows the read-only Rustup directory and missing dependencies. Prepare the toolchain, restore the read-only Rust cache, fetch locked main-branch dependencies, and install `cargo-nextest` before fetching or checking out the pull request so PR-controlled Cargo configuration stays outside trusted setup. Keep the same pinned `cargo-nextest` action available to the security-review job. Co-authored-by: Zanie Blue <contact@zanie.dev>