Add `rust-version = 1.81` and fmt toml files and imports.

[bheylin]

• Replace #[allow] with #[expect] (feature stabilized in Rust 1.81)
• Add rust_version = 1.81
• Format *.toml files
• Format *.rs file imports
• Add cargo_fmt_with_imports.sh to save the command used to format imports (needs +nightly)