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/features/src/lib.rs

@@ -862,8 +862,8 @@ pub const FEATURES: &[FeatureSpec] = &[
     FeatureSpec {
         id: Feature::UnifiedExecZshFork,
         key: "unified_exec_zsh_fork",
-        stage: Stage::UnderDevelopment,
-        default_enabled: false,
+        stage: Stage::Removed,
+        default_enabled: true,
     },
     FeatureSpec {
         id: Feature::ShellSnapshot,