Standardize shell execution on unified exec (#39757)

## What changed

- Remove the legacy `shell_command` handler and runtime, leaving `exec_command`
  and `write_stdin` as the shell execution tools.
- Treat legacy `shell_command` model metadata as `unified_exec`, and normalize
  legacy user opt-outs so they do not disable command execution. Managed feature
  requirements and `shell_tool` can still disable it.
- Preserve shell approvals, sandboxing, zsh-fork support, and output truncation
  through the unified execution path.

## Testing

- Cover legacy configuration and model-metadata compatibility.
- Exercise unified shell execution, approvals, truncation, and `apply_patch`
  serialization across the app-server and core test suites.

GitOrigin-RevId: 5c2fd6164fc3519cdae4944cb9db276b8467311c

codex-rs/core/src/exec.rs

@@ -205,6 +205,7 @@ impl ExecExpiration {
     }
 
     /// If ExecExpiration is a timeout, returns the timeout in milliseconds.
+    #[cfg(target_os = "windows")]
     pub(crate) fn timeout_ms(&self) -> Option<u64> {
         match self {
             ExecExpiration::Timeout(duration) => Some(duration.as_millis() as u64),