fix(streaming): artifact data protocol key

[sixlive]

Description

DataProtocolAdapter::handleArtifact() emitted artifact events with type: "artifact", which fails Zod validation in @ai-sdk/react v2's useChat. The AI SDK only accepts custom data parts with a data- prefix.

Changes

• Changed artifact event type from artifact to data-artifact
• Nested the payload (toolCallId, toolName, artifact) under a data key, matching the convention already used by handleDefault()
• Updated Vercel AI SDK documentation example to reflect the new format
• Added test verifying the data-artifact type and nested structure

Breaking Change

Consumers parsing the raw stream need to update artifact handling:

- if (data.type === 'artifact') {
-     artifacts.set(data.toolCallId, data.artifact);
+ if (data.type === 'data-artifact') {
+     artifacts.set(data.data.toolCallId, data.data.artifact);
  }