feat: support gpt-image-2 widescreen ratios

[yujiaxu-ops]

Summary

• allow gpt-image-2 with --aspect-ratio 16:9 and --aspect-ratio 9:16
• update shared text-to-image/image-to-image help to advertise all five supported GPT Image 2 ratios
• continue rejecting unsupported 4:3 and 3:4 combinations before credits are spent

This matches the public API support added in meshyd PR #5461.

Tests

• pnpm exec tsx --test tests/image-models.test.ts tests/surface.test.ts (21 passed)
• pnpm typecheck
• pnpm test (363 passed)
• pnpm build
• verified text-to-image and image-to-image create --help output