feat(ai): add Z.ai image generation

[opencode-agent]

Summary

• add a first-class ZAI.configure(...).image(modelID) facade with ZAI_API_KEY bearer fallback and provider export
• reuse the image HTTP transport/parser while keeping OpenAI and Z.ai request schemas, response schemas, and protocol-owned overlay fields isolated
• validate Z.ai userID length and content-filter roles/levels from the official contract without restricting model IDs or image sizes to a catalog
• document Z.ai image configuration and the 30-day temporary output URL lifetime
• add deterministic protocol coverage and a redacted live cassette

Live evidence

• recorded the international https://api.z.ai/api/paas/v4/images/generations route
• downloaded and decoded the returned image: valid 1024x1024, 8-bit sRGB JPEG
• visually inspected the result and confirmed the requested centered red circle on a white background
• scanned the cassette for API key and authorization material, then replayed it successfully

Tests

• bun x prettier --check ...
• bun typecheck
• bun run build
• bun test --timeout 30000 --only-failures (356 passed, 29 skipped)
• RECORDED_PROVIDER=zai bun test test/provider/zai-images.recorded.test.ts --timeout 30000
• git diff --check
• bounded secret scan of changed files and the Z.ai cassette

CI attribution

• the prior Linux e2e failure was unrelated: packages/app/e2e/regression/review-line-comment.spec.ts could not obtain line-number DOM bounds after retries
• this PR changes only packages/ai; the prior Linux AI unit job passed

Limitations

• international Z.ai endpoint and credentials only
• direct hosted text-to-image generation only; no hosted editing or conversational tools
• model selectors and image sizes remain structurally validated rather than catalog-restricted
• Z.ai output URLs are temporary and expire after 30 days; callers must persist images they need to retain

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