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/models-manager/src/model_info.rs

@@ -145,7 +145,7 @@ pub fn model_info_from_slug(slug: &str) -> ModelInfo {
         description: None,
         default_reasoning_level: None,
         supported_reasoning_levels: Vec::new(),
-        shell_type: ConfigShellToolType::Default,
+        shell_type: ConfigShellToolType::UnifiedExec,
         visibility: ModelVisibility::None,
         supported_in_api: true,
         priority: 99,