clean up some string handling cases

[davidhewitt]

Change Summary

• Use &str instead of Cow<'_, str> in a couple places where it's good enough
• Avoid some potential unsafety in converting a bytearray to a string

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 #1381 will not alter performance

_{Comparing dh/string-tidys (90dfb74) with main (40b8a94)}

Summary

✅ 155 untouched benchmarks

[sydney-runkle]

Looks great!

I just learned more about &str and Cow<'_, str>. This was easy to review - thanks!