refactor: apply clippy::io_other_error

[shulaoda]

Summary

• Replaces io::Error::new(io::ErrorKind::Other, ..) with
  io::Error::other(..) across:
  • src/cargo-fmt/main.rs (3 sites)
  • src/config/mod.rs (1 site)
  • src/format-diff/main.rs (1 site)
• Addresses clippy::io_other_error, run as a single-rule pass:
  cargo clippy -- -A clippy::all -W clippy::io_other_error.

[ytmimi]

Going to close until we decide on which clippy lints we want to enforce in the codebase.