fix: preserve platform-specific core shell env vars (#16707)

## Why

We were seeing failures in the following tests as part of trying to get
all the tests running under Bazel on Windows in CI
(https://github.com/openai/codex/pull/16528):

```
suite::shell_command::unicode_output::with_login
suite::shell_command::unicode_output::without_login
```

Certainly `PATHEXT` should have been included in the extra `CORE_VARS`
list, so we fix that up here, but also take things a step further for
now by forcibly ensuring it is set on Windows in the return value of
`create_env()`. Once we get the Windows Bazel build working reliably
(i.e., after #16528 is merged), we should come back to this and confirm
we can remove the special case in `create_env()`.

## What

- Split core env inheritance into `COMMON_CORE_VARS` plus
platform-specific allowlists for Windows and Unix in
[`exec_env.rs`](https://github.com/openai/codex/blob/1b55c88fbf585b32cd553cb9d02ec817f2ad6ebc/codex-rs/core/src/exec_env.rs#L45-L81).
- Preserve `PATHEXT`, `USERNAME`, and `USERPROFILE` on Windows, and
`HOME` / locale vars on Unix.
- Backfill a default `PATHEXT` in `create_env()` on Windows if the
parent env does not provide one, so child process launch still works in
stripped-down Bazel environments.
- Extend the Windows exec-env test to assert mixed-case `PathExt`
survives case-insensitive core filtering, and document why the
shell-command Unicode test goes through a child process.

## Verification

- `cargo test -p codex-core exec_env::tests`

codex-rs/core/src/exec_env_tests.rs

@@ -1,6 +1,7 @@
 use super::*;
 use codex_config::types::ShellEnvironmentPolicyInherit;
 use maplit::hashmap;
+use pretty_assertions::assert_eq;
 
 fn make_vars(pairs: &[(&str, &str)]) -> Vec<(String, String)> {
     pairs
@@ -171,6 +172,7 @@ fn test_inherit_all_with_default_excludes() {
 fn test_core_inherit_respects_case_insensitive_names_on_windows() {
     let vars = make_vars(&[
         ("Path", "C:\\Windows\\System32"),
+        ("PathExt", ".COM;.EXE;.BAT;.CMD"),
         ("TEMP", "C:\\Temp"),
         ("FOO", "bar"),
     ]);
@@ -185,6 +187,7 @@ fn test_core_inherit_respects_case_insensitive_names_on_windows() {
     let result = populate_env(vars, &policy, Some(thread_id));
     let mut expected: HashMap<String, String> = hashmap! {
         "Path".to_string() => "C:\\Windows\\System32".to_string(),
+        "PathExt".to_string() => ".COM;.EXE;.BAT;.CMD".to_string(),
         "TEMP".to_string() => "C:\\Temp".to_string(),
     };
     expected.insert(CODEX_THREAD_ID_ENV_VAR.to_string(), thread_id.to_string());
@@ -192,6 +195,47 @@ fn test_core_inherit_respects_case_insensitive_names_on_windows() {
     assert_eq!(result, expected);
 }
 
+#[test]
+#[cfg(target_os = "windows")]
+fn create_env_inserts_pathext_on_windows_when_missing() {
+    let vars = make_vars(&[]);
+
+    let policy = ShellEnvironmentPolicy {
+        inherit: ShellEnvironmentPolicyInherit::None,
+        ignore_default_excludes: true,
+        ..Default::default()
+    };
+
+    let result = create_env_from_vars(vars, &policy, /*thread_id*/ None);
+
+    let expected: HashMap<String, String> = hashmap! {
+        "PATHEXT".to_string() => ".COM;.EXE;.BAT;.CMD".to_string(),
+    };
+    assert_eq!(result, expected);
+}
+
+#[test]
+#[cfg(target_os = "windows")]
+fn create_env_preserves_existing_pathext_case_insensitively_on_windows() {
+    let vars = make_vars(&[("PathExt", ".COM;.EXE;.BAT;.CMD;.PS1")]);
+
+    let policy = ShellEnvironmentPolicy {
+        inherit: ShellEnvironmentPolicyInherit::Core,
+        ignore_default_excludes: true,
+        ..Default::default()
+    };
+
+    let result = create_env_from_vars(vars, &policy, /*thread_id*/ None);
+
+    let pathext_vars = result
+        .iter()
+        .filter(|(key, _)| key.eq_ignore_ascii_case("PATHEXT"))
+        .collect::<Vec<_>>();
+
+    assert_eq!(pathext_vars.len(), 1);
+    assert_eq!(pathext_vars[0].1, ".COM;.EXE;.BAT;.CMD;.PS1");
+}
+
 #[test]
 fn test_inherit_none() {
     let vars = make_vars(&[("PATH", "/usr/bin"), ("HOME", "/home")]);