feat: add widgetSessionId to spec for widget consolidation

[ochafik]

Summary

Adds widgetSessionId to the MCP Apps specification, enabling widget consolidation where only the latest widget with a given session ID is displayed.

Changes

• Added McpUiToolResultMeta interface with widgetSessionId field in spec.types.ts
• Documented "Widget Session Consolidation" in the specification (apps.mdx)
• Generated updated schemas

Specification

When multiple tool results share the same widgetSessionId, the host:

1. Finds earlier widgets with the same sessionId
2. Calls ui/resource-teardown on superseded widgets
3. Hides superseded widgets from the UI
4. Only displays the latest widget

Example

{
  content: [{ type: "text", text: "Cart updated" }],
  structuredContent: { items: [...] },
  _meta: {
    ui: {
      widgetSessionId: "user-cart-abc123"
    }
  }
}

Related

• Host implementation: https://github.com/anthropics/apps/pull/19158

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commit: 225d9cf