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/analytics/src/client_tests.rs

@@ -588,6 +588,7 @@ fn sample_thread_archive_request() -> ClientRequest {
 
 fn sample_thread(thread_id: &str) -> Thread {
     Thread {
+        environments: None,
         id: thread_id.to_string(),
         extra: None,
         session_id: format!("session-{thread_id}"),