Expose structured standalone web search results

[copyberry]

Expose structured standalone web search results

Why

Standalone web search can return structured result DTOs separately from its
model-facing text output. App-server clients need access to that data without
coupling Codex to every result type or field.

What changed

• Preserve the optional results array from /v1/alpha/search as opaque JSON
  through web-search completion events, thread history, and app-server
  webSearch items.
• Keep responses from older endpoints compatible when results is absent, and
  allow clients to ignore result types and fields they do not understand.
• Regenerate the protocol schemas and TypeScript definitions for the optional
  field.

Testing

• Cover responses with missing, empty, and forward-compatible result payloads.
• Verify standalone web-search results round-trip through app-server item
  notifications and thread-item conversion.

[github-actions]

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