[Canva] prepare-design-generation returns no UI/HTML in Codex Windows app

Environment:

• Codex Windows app
• Canva connector plugin version: 1.0.2
• Date/time: 2026-06-22, Asia/Shanghai

Steps to reproduce:

1. Ask Canva to generate a visual design.
2. Codex calls prepare-design-generation.

Expected:
The Canva visual-reference / design-generation widget renders in the conversation.

Actual:
Codex displays: “MCP 应用未返回 HTML 内容” / “MCP app did not return HTML content”.

Observed tool result:

• isError: false
• textual content only: “Action completed.”
• no renderable HTML, widget resource, resource_link, or UI metadata returned.

Additional evidence:

• Canva generate-design succeeds and returns candidate designs.
• Therefore authorization and the generation backend work; only the

prepare-design-generation UI render response is missing.