feat(ai): add Z.ai image generation

[opencode-agent]

Summary

• add ZAI.configure(...).image(modelID) with ZAI_API_KEY bearer fallback, configurable auth/base URL/headers/HTTP overlays, and arbitrary model IDs
• infer open request-scoped Z.ai options while preserving known value kinds, arbitrary native fields, and future string values
• lower userID to user_id, let raw native fields override aliases, and give http.body final precedence
• decode provider output structurally without local model/value compatibility rejections
• represent hosted URL results as application/octet-stream because Z.ai does not return trustworthy MIME metadata
• preserve the merged Google image documentation and add concise Z.ai usage plus the 30-day URL expiry warning

API

Image.generate({
  model: ZAI.configure({ apiKey }).image("any-model-id"),
  prompt,
  options: {
    quality: "hd",
    userID: "user-123",
    future_option: true,
  },
  http,
})

Tests

• bun x prettier --check README.md src/protocols/zai-images.ts src/providers/index.ts src/providers/zai.ts test/image.types.ts test/provider/zai-images.test.ts test/provider/zai-images.recorded.test.ts test/fixtures/recordings/zai-images/generates-an-image.json
• bun typecheck
• bun run build
• bun test --timeout 30000 --only-failures (386 passed, 29 skipped)
• RECORDED_PROVIDER=zai bun test test/provider/zai-images.recorded.test.ts --timeout 30000 --only-failures (1 passed)
• git diff --check origin/v2
• bounded credential-pattern scan of the PR diff
• clean git merge-tree --write-tree origin/v2 HEAD

Requested by: @rekram1-node (Aiden Cline via Slack)