AG-UI support for nested events

In recent trends, multi-agent collaboration and parallel processing have become increasingly important for handling complex tasks. The current AG-UI design only supports sequential mode and lacks support for parallel events, which can lead to event confusion.

My preliminary idea is to add an `id` and `parent_id` to message, tool_call, and step events, allowing them to be associated across arbitrary hierarchical levels.

Similar issues: #174 #87