Enable unified exec by default on Windows (#38625)

## What changed

- Enable the stable `unified_exec` feature by default on every platform.
- Update cross-platform integration test expectations so `exec_command` and
  `write_stdin` are exposed on Windows instead of `shell_command`.

GitOrigin-RevId: e5c864bacbde7eab109e7a0e399b8f7b843b384c

codex-rs/core/tests/suite/code_mode.rs

@@ -4958,7 +4958,7 @@ text(JSON.stringify({
     assert_eq!(
         parsed,
         serde_json::json!({
-            "hasExecCommand": !cfg!(windows),
+            "hasExecCommand": true,
             "hasNamespacedEcho": true,
         })
     );

codex-rs/core/tests/suite/prompt_caching.rs

@@ -182,11 +182,7 @@ async fn prompt_tools_are_consistent_across_requests() -> anyhow::Result<()> {
         .await?;
     wait_for_event(&codex, |ev| matches!(ev, EventMsg::TurnComplete(_))).await;
 
-    let mut expected_tools_names = if cfg!(windows) {
-        vec!["shell_command"]
-    } else {
-        vec!["exec_command", "write_stdin"]
-    };
+    let mut expected_tools_names = vec!["exec_command", "write_stdin"];
     expected_tools_names.extend([
         "update_plan",
         "request_user_input",

codex-rs/features/src/lib.rs

@@ -826,7 +826,7 @@ pub const FEATURES: &[FeatureSpec] = &[
         id: Feature::UnifiedExec,
         key: "unified_exec",
         stage: Stage::Stable,
-        default_enabled: !cfg!(windows),
+        default_enabled: true,
     },
     FeatureSpec {
         id: Feature::ShellZshFork,