[codex] Remove unused Rust helpers (#17146)

## Summary

Removes high-confidence unused Rust helper functions and exports across
`codex-tui`, `codex-shell-command`, and utility crates.

The cleanup includes dead TUI helper methods, unused
path/string/elapsed/fuzzy-match utilities, an unused Windows PowerShell
lookup helper, and the unused terminal palette version counter. This
keeps the remaining public surface smaller without changing behavior.

## Validation

- `just fmt`
- `cargo test -p codex-tui -p codex-shell-command -p codex-utils-elapsed
-p codex-utils-fuzzy-match -p codex-utils-string -p codex-utils-path`
- `just fix -p codex-tui -p codex-shell-command -p codex-utils-elapsed
-p codex-utils-fuzzy-match -p codex-utils-string -p codex-utils-path`
- `git diff --check`

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

@@ -23,28 +23,6 @@ pub fn take_bytes_at_char_boundary(s: &str, maxb: usize) -> &str {
     &s[..last_ok]
 }
 
-// Take a suffix of a &str within a byte budget at a char boundary
-#[inline]
-pub fn take_last_bytes_at_char_boundary(s: &str, maxb: usize) -> &str {
-    if s.len() <= maxb {
-        return s;
-    }
-    let mut start = s.len();
-    let mut used = 0usize;
-    for (i, ch) in s.char_indices().rev() {
-        let nb = ch.len_utf8();
-        if used + nb > maxb {
-            break;
-        }
-        start = i;
-        used += nb;
-        if start == 0 {
-            break;
-        }
-    }
-    &s[start..]
-}
-
 /// Sanitize a tag value to comply with metric tag validation rules:
 /// only ASCII alphanumeric, '.', '_', '-', and '/' are allowed.
 pub fn sanitize_metric_tag_value(value: &str) -> String {