fix(chatgpt): ignore image placeholders and upload previews

[ken-zy]

Summary

• Filter ChatGPT user-uploaded reference previews from image output detection
• Avoid saving transparent/blank canvas placeholders as successful image results
• Add regression coverage for upload-preview filtering and canvas content sampling

Test Plan

• npm test -- --run clis/chatgpt/utils.test.js
• npm run test:adapter -- --run clis/chatgpt
• npm run typecheck

Background

Verified locally against ChatGPT Web image generation: before this patch the command could report success while saving either a blank transparent canvas placeholder or the uploaded reference preview instead of the generated image. After filtering these candidates, the adapter saved the real generated output.