docs: A2A send/get task documentation

[dat-adi]

• Added in documentation for client-server interaction with the A2A agent, using the A2AClient and Message object.
• Addressed send_task and get_task.
• Added content on in-progress and completed tasks on the side of the provider.

[hyperlint-ai]

PR Change Summary

Enhanced documentation for A2A client-server interaction, focusing on task management.

• Added documentation for using the A2AClient to send and get tasks.
• Included examples for sending tasks and retrieving their status.
• Documented handling of in-progress and completed tasks.

Modified Files

• docs/a2a.md

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[dat-adi]

Resolves #1726