Standardize shell execution on unified exec (#39772)

## What changed

- Use `exec_command` and `write_stdin` as the shell tool surface.
- Treat legacy `default`, `local`, and `shell_command` model metadata as `unified_exec`.
- Remove obsolete shell-selection configuration and runtime paths while preserving the feature and policy gates for zsh fork execution.

GitOrigin-RevId: d743cbe598630d73052f1fecad680c4cde17977d

codex-rs/core/src/exec.rs

@@ -161,14 +161,6 @@ pub enum ExecExpirationOutcome {
     Cancelled,
 }
 
-impl From<Option<u64>> for ExecExpiration {
-    fn from(timeout_ms: Option<u64>) -> Self {
-        timeout_ms.map_or(ExecExpiration::DefaultTimeout, |timeout_ms| {
-            ExecExpiration::Timeout(Duration::from_millis(timeout_ms))
-        })
-    }
-}
-
 impl From<u64> for ExecExpiration {
     fn from(timeout_ms: u64) -> Self {
         ExecExpiration::Timeout(Duration::from_millis(timeout_ms))