Move string truncation helpers into codex-utils-string (#15572)

- move the shared byte-based middle truncation logic from `core` into
`codex-utils-string`
- keep token-specific truncation in `codex-core` so rollout can reuse
the shared helper in the next stacked PR

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/utils/string/src/lib.rs

@@ -1,3 +1,11 @@
+mod truncate;
+
+pub use truncate::approx_bytes_for_tokens;
+pub use truncate::approx_token_count;
+pub use truncate::approx_tokens_from_byte_count;
+pub use truncate::truncate_middle_chars;
+pub use truncate::truncate_middle_with_token_budget;
+
 // Truncate a &str to a byte budget at a char boundary (prefix)
 #[inline]
 pub fn take_bytes_at_char_boundary(s: &str, maxb: usize) -> &str {
@@ -112,6 +120,7 @@ fn parse_markdown_hash_location_point(point: &str) -> Option<(&str, Option<&str>
 }
 
 #[cfg(test)]
+#[allow(warnings, clippy::all)]
 mod tests {
     use super::find_uuids;
     use super::normalize_markdown_hash_location_suffix;