fix(xai): fold namespace tools and restore dispatcher tool calls

- Introduce `xaiNamespaceRestorer` to track and restore folded dispatcher tool calls across SSE and WebSocket response events.
- Support unwrapping dispatcher tool calls and arguments in `output_item.added` and `function_call_arguments.done` events.
- Normalize historical input namespace tool calls to dispatcher format when namespace folding is active.

Closes: #5214