Add wait_s and wait_for tools for async workflows

[erikgaas]

Summary

Adds two new tools for async prompt workflows:

• wait_s(seconds) — Simple async sleep
• wait_for(msg_id, timeout=10) — Poll until message finishes running, return its output

Use case

Enables workflows where one prompt spawns other prompts/code cells and waits for their results:

msg_id = _add_msg_unsafe('Say hello', msg_type='prompt', run=True)
output = await wait_for(msg_id)
print(f'Got: {output}')

Used with solveit's async prompts feature (PR #1439).