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/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,