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/backend-client/src/lib.rs

@@ -8,6 +8,9 @@ pub use types::AccountEntry;
 pub use types::AccountsCheckResponse;
 pub use types::CodeTaskDetailsResponse;
 pub use types::CodeTaskDetailsResponseExt;
+pub use types::CodexWorkspaceMessage;
+pub use types::CodexWorkspaceMessageType;
+pub use types::CodexWorkspaceMessagesResponse;
 pub use types::ConfigBundleResponse;
 pub use types::ConsumeRateLimitResetCreditCode;
 pub use types::ConsumeRateLimitResetCreditResponse;