Report selected environment connection transitions (#33251)

## What changed

- Track connected and disconnected states across initial remote exec-server connections and reconnection attempts.
- Emit experimental `thread/environment/connected` and `thread/environment/disconnected` app-server notifications for each thread selecting the environment. Each payload identifies the thread and environment; current state is not replayed when a thread starts.
- Stop forwarding connection events when an environment selection is removed or replaced.

## Testing

- Cover connection, disconnection, reconnection, shared-environment notifications, and replacement of a selected environment.

GitOrigin-RevId: 5dd767372363c4a2a8319fc16164be117d5bd20c

codex-rs/app-server-protocol/schema/json/ServerNotification.json

@@ -1140,6 +1140,21 @@
       ],
       "type": "string"
     },
+    "EnvironmentConnectionNotification": {
+      "properties": {
+        "environmentId": {
+          "type": "string"
+        },
+        "threadId": {
+          "type": "string"
+        }
+      },
+      "required": [
+        "environmentId",
+        "threadId"
+      ],
+      "type": "object"
+    },
     "ErrorNotification": {
       "properties": {
         "error": {
@@ -5991,6 +6006,46 @@
       "title": "Thread/goal/clearedNotification",
       "type": "object"
     },
+    {
+      "properties": {
+        "method": {
+          "enum": [
+            "thread/environment/connected"
+          ],
+          "title": "Thread/environment/connectedNotificationMethod",
+          "type": "string"
+        },
+        "params": {
+          "$ref": "#/definitions/EnvironmentConnectionNotification"
+        }
+      },
+      "required": [
+        "method",
+        "params"
+      ],
+      "title": "Thread/environment/connectedNotification",
+      "type": "object"
+    },
+    {
+      "properties": {
+        "method": {
+          "enum": [
+            "thread/environment/disconnected"
+          ],
+          "title": "Thread/environment/disconnectedNotificationMethod",
+          "type": "string"
+        },
+        "params": {
+          "$ref": "#/definitions/EnvironmentConnectionNotification"
+        }
+      },
+      "required": [
+        "method",
+        "params"
+      ],
+      "title": "Thread/environment/disconnectedNotification",
+      "type": "object"
+    },
     {
       "properties": {
         "method": {