fix: fall back to inline image data for internal URIs

[kolodziejm]

Summary

• Fall back to inline ACP image data when an image block carries an internal or otherwise non-fetchable URI.
• Keep existing behavior for http:, https:, and data: image URLs.
• Add a regression test covering zed:///agent/pasted-image?... input.

Motivation

Some ACP clients attach pasted images with inline data/mimeType plus a client-internal uri. For example, Zed can send a pasted image URI such as zed:///agent/pasted-image?name=Image. The adapter currently forwards that URI to Codex as an image URL, which later becomes an OpenAI image_url value and is rejected because zed:///... is not a valid URL for the API.

Using the inline image payload for non-fetchable URI schemes preserves image input while still allowing real external URLs and already-inline data: URLs to pass through unchanged.

Validation

• npm test -- src/__tests__/CodexACPAgent/CodexAcpClient.test.ts
• npm run typecheck

[benbrandt]

Thanks!