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/app-server-protocol/schema/json/v2/ThreadStartedNotification.json

@@ -1276,6 +1276,29 @@
       ],
       "type": "string"
     },
+    "ThreadEnvironment": {
+      "description": "An environment selected by a loaded thread, independent of connection status.",
+      "properties": {
+        "cwd": {
+          "$ref": "#/definitions/LegacyAppPathString"
+        },
+        "environmentId": {
+          "type": "string"
+        },
+        "runtimeWorkspaceRoots": {
+          "items": {
+            "$ref": "#/definitions/LegacyAppPathString"
+          },
+          "type": "array"
+        }
+      },
+      "required": [
+        "cwd",
+        "environmentId",
+        "runtimeWorkspaceRoots"
+      ],
+      "type": "object"
+    },
     "ThreadExtra": {
       "description": "Extra app-server data for a thread.",
       "type": "object"