Add workspace messages app-server API (#29001)

## Summary

- Add backend-client types and fetch support for active workspace
messages.
- Add the app-server v2 `account/workspaceMessages/read` method,
generated schemas, and README documentation.
- Delegate workspace-message eligibility to the Codex backend feature
gate; map a backend 404 to `featureEnabled: false`.

## Testing

- `just write-app-server-schema`
- `just test -p codex-backend-client`
- `just test -p codex-app-server-protocol`
- `just test -p codex-app-server workspace_messages`
- `just fix -p codex-backend-client -p codex-app-server-protocol -p
codex-app-server`
- `just fmt`

## Stack

- Base PR for #28232, which adds the TUI status-line integration.

codex-rs/app-server-protocol/src/protocol/common.rs

@@ -1022,6 +1022,12 @@ client_request_definitions! {
         response: v2::GetAccountTokenUsageResponse,
     },
 
+    GetWorkspaceMessages => "account/workspaceMessages/read" {
+        params: #[ts(type = "undefined")] #[serde(skip_serializing_if = "Option::is_none")] Option<()>,
+        serialization: None,
+        response: v2::GetWorkspaceMessagesResponse,
+    },
+
     SendAddCreditsNudgeEmail => "account/sendAddCreditsNudgeEmail" {
         params: v2::SendAddCreditsNudgeEmailParams,
         serialization: global("account-auth"),
@@ -2541,6 +2547,24 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn serialize_get_workspace_messages() -> Result<()> {
+        let request = ClientRequest::GetWorkspaceMessages {
+            request_id: RequestId::Integer(1),
+            params: None,
+        };
+        assert_eq!(request.id(), &RequestId::Integer(1));
+        assert_eq!(request.method(), "account/workspaceMessages/read");
+        assert_eq!(
+            json!({
+                "method": "account/workspaceMessages/read",
+                "id": 1,
+            }),
+            serde_json::to_value(&request)?,
+        );
+        Ok(())
+    }
+
     #[test]
     fn serialize_client_response() -> Result<()> {
         let cwd = absolute_path("/tmp");