[Widget] Render agent rich content

[giannagerton]

Adds the widget-side consumer of the onRichContent SDK callback (#912). Adds rich content rendering to the chat widget. The agent can display components inline in the transcript starting with quick reply buttons. It's inert on merge, since nothing on the server emits the event yet.

Starting with buttons rather than cards, for two reasons:

• ButtonGroup is the primitive other components build on. A card or carousel renders its own buttons through the same component.
• Going through the customer analysis, clickable links and guided options came up far more often than product cards.

Button types

Type	Behaviour
message	Sends its text as the user's own turn
link	Opens its url in a new tab

Max 3 buttons per row. Buttons that can run actions directly to follow.

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Note

Medium Risk
New user-facing transcript paths and outbound links/message sends, mitigated by strict https-only link validation and safe fallbacks for bad payloads.

Overview
Adds inline rich content in the convai widget transcript, wired from the SDK onRichContent callback into new rich_content transcript entries and display rows.

The first supported component is buttons: message buttons call sendUserMessage with the payload text; link buttons use an extended Button (as="a", outline, target="_blank"). Props are validated with zod/mini (caps, trimming, https-only links); unknown or invalid payloads show rich_content_unavailable instead of breaking the UI.

Display transcript behavior is covered by new tests so rich content stays in arrival order relative to agent replies, tool-status placeholders, and the typing indicator.

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[kamilk-11]

Nice!