Goal

Ensure Rust integration tests use the intended rustup-managed toolchain instead of the Nix rustc wrapper.

Why

PR #5711 bumped the MSRV, but CI continued failing with errors indicating rustc 1.86.0 was being used despite a newer toolchain being installed via rustup. In Nix-based devShells, Cargo can still resolve to the Nix rustc wrapper unless the rustup toolchain is explicitly prioritized in the dev shell. This PR removes the previous installation via pkgs.rustc / pkgs.cargo in favor of the rustup-driven toolchain installation to keep Rust versions aligned and up to date.

How

• Rust toolchain selection is now determined via nix/rust-toolchain.toml
• Ensure rustup-installed cargo/rustc take precedence in the devShell PATH
• Remove reliance on the Nix package version of rust
• Simplify rust_test to rely on rustup’s implicit toolchain resolution (no explicit installation needed anymore!)

Callouts

• This PR unblocks chore: bump msrv #5711

Testing

Rebased on #5711 and ran rust_test locally inside of nix and confirmed tests pass

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.