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/src/protocol/common.rs

@@ -1630,6 +1630,10 @@ server_notification_definitions! {
     ThreadNameUpdated => "thread/name/updated" (v2::ThreadNameUpdatedNotification),
     ThreadGoalUpdated => "thread/goal/updated" (v2::ThreadGoalUpdatedNotification),
     ThreadGoalCleared => "thread/goal/cleared" (v2::ThreadGoalClearedNotification),
+    #[experimental("thread/environment/connected")]
+    EnvironmentConnected => "thread/environment/connected" (v2::EnvironmentConnectionNotification),
+    #[experimental("thread/environment/disconnected")]
+    EnvironmentDisconnected => "thread/environment/disconnected" (v2::EnvironmentConnectionNotification),
     #[experimental("thread/settings/updated")]
     ThreadSettingsUpdated => "thread/settings/updated" (v2::ThreadSettingsUpdatedNotification),
     ThreadTokenUsageUpdated => "thread/tokenUsage/updated" (v2::ThreadTokenUsageUpdatedNotification),

codex-rs/app-server-protocol/src/protocol/v2/environment.rs

@@ -21,6 +21,14 @@ pub struct EnvironmentAddParams {
 #[ts(export_to = "v2/")]
 pub struct EnvironmentAddResponse {}
 
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(rename_all = "camelCase", export_to = "v2/")]
+pub struct EnvironmentConnectionNotification {
+    pub thread_id: String,
+    pub environment_id: String,
+}
+
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
 #[serde(rename_all = "camelCase")]
 #[ts(export_to = "v2/")]