Expose loaded thread environments in app-server responses (#42386)

## What changed

- Add the experimental `Thread.environments` field with each selected environment's ID, working directory, and runtime workspace roots.
- Populate the field from live thread state in start, resume, read, list, and unarchive responses. Use `null` for unloaded threads and an empty list when a loaded thread has no environment selection.
- Document that the field reports selection rather than connection status and does not affect executor selection or resume behavior.

## Testing

- Cover selection updates, unloaded and resumed threads, loaded pathless unarchives, and clients that subscribe after environment connection notifications.

GitOrigin-RevId: a6daa06aff7089d7c5a1e4c7d03109b41f12fa3c

codex-rs/exec/src/lib_tests.rs

@@ -351,6 +351,7 @@ async fn resume_lookup_model_providers_filters_only_last_lookup() {
 #[test]
 fn turn_items_for_thread_returns_matching_turn_items() {
     let thread = AppServerThread {
+        environments: None,
         id: "thread-1".to_string(),
         extra: None,
         session_id: "thread-1".to_string(),
@@ -841,6 +842,7 @@ async fn session_configured_from_thread_response_preserves_parent_thread_id() {
 fn sample_thread_start_response() -> ThreadStartResponse {
     ThreadStartResponse {
         thread: codex_app_server_protocol::Thread {
+            environments: None,
             id: "67e55044-10b1-426f-9247-bb680e5fe0c8".to_string(),
             extra: None,
             session_id: "67e55044-10b1-426f-9247-bb680e5fe0c7".to_string(),