Support ability for agent to show custom "suggested prompts" as part of UI responses

[jacobsimionato]

A potential customer wants to show suggested prompt chips - see internal Google doc: https://docs.google.com/document/d/1SWnXLH1s_R5zY1EwLHIukg0aI1XBTn55osCyG1Tol2E/edit?tab=t.0

I think we want to avoid too much special casing here, especially seeing as the client may want to show the suggested prompts using existing suggested prompt rendering logic, potentially outside the Gen UI surface area.

Option 1: Add a suggestedPrompts field to message at either the A2A or A2UI level

Perhaps the customer could include suggested prompts as a metadata param on the A2A Message object, or as a special Data Part. This way, it's a totally separate and optional system to A2UI surfaces and components.

Option 2: Add a 'suggestedPrompt' usageHint to the "button" Component, and perhaps a "suggestedPrompt" action type?

This could allow Buttons to be styled as suggested prompts. There would still be the full button functionality available, e.g. ability to include custom context or references to data model values in the message, which could be useful.

This would let the agent display suggested prompts within surfaces, and potentially nested within other UI components, which could be handy.