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/codex-api/tests/models_integration.rs

@@ -71,7 +71,7 @@ async fn models_client_hits_models_endpoint() {
                     description: ReasoningEffort::High.to_string(),
                 },
             ],
-            shell_type: ConfigShellToolType::ShellCommand,
+            shell_type: ConfigShellToolType::UnifiedExec,
             visibility: ModelVisibility::List,
             supported_in_api: true,
             priority: 1,