[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/shell-command/src/powershell.rs

@@ -68,19 +68,6 @@ pub fn extract_powershell_command(command: &[String]) -> Option<(&str, &str)> {
     None
 }
 
-/// This function attempts to find a valid PowerShell executable on the system.
-/// It first tries to find pwsh.exe, and if that fails, it tries to find
-/// powershell.exe.
-#[cfg(windows)]
-#[allow(dead_code)]
-pub(crate) fn try_find_powershellish_executable_blocking() -> Option<AbsolutePathBuf> {
-    if let Some(pwsh_path) = try_find_pwsh_executable_blocking() {
-        Some(pwsh_path)
-    } else {
-        try_find_powershell_executable_blocking()
-    }
-}
-
 /// This function attempts to find a powershell.exe executable on the system.
 pub fn try_find_powershell_executable_blocking() -> Option<AbsolutePathBuf> {
     try_find_powershellish_executable_in_path(&["powershell.exe"])