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/schema/json/ClientRequest.json

@@ -6462,6 +6462,29 @@
       "title": "Account/usage/readRequest",
       "type": "object"
     },
+    {
+      "properties": {
+        "id": {
+          "$ref": "#/definitions/RequestId"
+        },
+        "method": {
+          "enum": [
+            "account/workspaceMessages/read"
+          ],
+          "title": "Account/workspaceMessages/readRequestMethod",
+          "type": "string"
+        },
+        "params": {
+          "type": "null"
+        }
+      },
+      "required": [
+        "id",
+        "method"
+      ],
+      "title": "Account/workspaceMessages/readRequest",
+      "type": "object"
+    },
     {
       "properties": {
         "id": {