[codex] preserve explicit environment cwd (#27995)

## Why

`TurnEnvironmentSelections::new` rewrote the primary environment's
explicit `cwd` to the legacy fallback cwd. For a remote-first selection,
this could replace the remote working directory with a local fallback
path and made the legacy cwd overlay authoritative over
environment-owned state.

## What changed

- Preserve every explicit environment cwd when constructing turn
environment selections.
- Keep `cwd`-only app-server updates compatible by rebuilding the
default environment selections at the requested cwd.
- Cover both explicit primary cwd preservation and cwd-only updates
reaching the model-visible execution environment.

## Testing

- `just test -p codex-core
session_update_settings_does_not_rewrite_sticky_environment_cwds`
- `just test -p codex-core
environment_settings_preserve_explicit_primary_cwd`
- `just test -p codex-app-server
thread_settings_update_cwd_retargets_default_environment`

codex-rs/core/src/session/tests.rs

@@ -4615,6 +4615,7 @@ async fn session_update_settings_does_not_rewrite_sticky_environment_cwds() {
             .environment_selections()
             .to_vec()
     };
+    let expected_environments = current_environments.clone();
     std::fs::create_dir_all(updated_cwd.as_path()).expect("create project dir");
 
     session
@@ -4628,21 +4629,28 @@ async fn session_update_settings_does_not_rewrite_sticky_environment_cwds() {
         .await
         .expect("cwd update should succeed");
 
-    let session_cwd = {
+    let (session_cwd, stored_environments) = {
         let state = session.state.lock().await;
-        state.session_configuration.cwd().clone()
+        (
+            state.session_configuration.cwd().clone(),
+            state
+                .session_configuration
+                .environment_selections()
+                .to_vec(),
+        )
     };
     let config = session.get_config().await;
     let next_turn = session.new_default_turn().await;
 
     assert_eq!(session_cwd, updated_cwd);
+    assert_eq!(stored_environments, expected_environments);
     #[allow(deprecated)]
     let turn_cwd = turn_context.cwd.clone();
     #[allow(deprecated)]
     let next_turn_cwd = next_turn.cwd.clone();
     assert_eq!(config.cwd, turn_cwd);
-    assert_eq!(next_turn_cwd, updated_cwd);
-    assert_eq!(next_turn.config.cwd, updated_cwd);
+    assert_eq!(next_turn_cwd, turn_cwd);
+    assert_eq!(next_turn.config.cwd, turn_cwd);
 }
 
 #[tokio::test]
@@ -4678,7 +4686,7 @@ async fn relative_cwd_update_without_environments_resolves_under_session_cwd() {
 }
 
 #[tokio::test]
-async fn cwd_update_rewrites_sticky_environment_cwd() {
+async fn environment_settings_preserve_explicit_primary_cwd() {
     let (session, _turn_context) = make_session_and_context().await;
     let (original_cwd, environment_cwd, environments) = {
         let mut state = session.state.lock().await;
@@ -4706,9 +4714,8 @@ async fn cwd_update_rewrites_sticky_environment_cwd() {
     assert_eq!(state.session_configuration.cwd(), &updated_cwd);
     assert_eq!(
         state.session_configuration.environment_selections()[0].cwd,
-        PathUri::from_abs_path(&updated_cwd)
+        PathUri::from_abs_path(&environment_cwd)
     );
-    assert_ne!(environment_cwd, updated_cwd);
 }
 
 #[tokio::test]