fix clippy lints for Rust 1.86

[davidhewitt]

Change Summary

Fix lints for Rust 1.86

NB use of the write!(string, ...) macro instead of string.push_str(&format!(...)) is recommended by clippy because it's more efficient (avoids an intermediate allocation in the format!() call), and should be infallible just like push_str but we have to handle the results because other possible targets for write!() (like IO streams) might fail.

Because it should never fail, I chose to just .expect() these.

Related issue number

N/A

Checklist

• Unit tests for the changes exist
• Documentation reflects the changes where applicable
• Pydantic tests pass with this pydantic-core (except for expected changes)
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

[codspeed-hq]

CodSpeed Performance Report

Merging #1682 will not alter performance

_{Comparing dh/rust-1.86 (9f90c3a) with main (baa8cbf)}

Summary

✅ 157 untouched benchmarks