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/src/message_processor.rs

@@ -1427,6 +1427,9 @@ impl MessageProcessor {
             ClientRequest::GetAccountTokenUsage { .. } => {
                 self.account_processor.get_account_token_usage().await
             }
+            ClientRequest::GetWorkspaceMessages { .. } => {
+                self.account_processor.get_workspace_messages().await
+            }
             ClientRequest::SendAddCreditsNudgeEmail { params, .. } => {
                 self.account_processor
                     .send_add_credits_nudge_email(params)