feat: add async multi-agent model example

[nisweesi]

Add async multi-agent model example

Checklist

• I have read both the CONTRIBUTING.md and Contributor License Agreement documents.
• I have created an issue or feature request and received approval from xAI maintainers. (minor changes like fixing typos can skip this step)
• I have tested my changes locally and they pass all CI checks.
• I have added necessary documentation or updated existing documentation.

Description

This PR adds a new asynchronous example demonstrating how to use the grok-4.20-multi-agent model with the xAI SDK.

The example shows how to:

• Configure the number of agents
• Enable web search and X search tools
• Submit a research prompt
• Use either chat.sample() or chat.stream()
• Display reasoning-token progress when enabled
• Print the final response and request cost

The purpose is to provide SDK users with a clear reference for running multi-agent research requests asynchronously.

Related Issue

If applicable, link to the related feature request or bug report issue (e.g., #123). If none, state "N/A".

Type of Change

• Bug fix
• New feature
• Documentation update
• Other (please specify)

Additional Notes

Add any other information or context that might be helpful for reviewers.