feat: add premise fallback to Vercel schema selection

[rlippmann]

Summary

Adds premise-driven fallback schema selection to the Vercel AI SDK generateObject schema-selection example.

The example still selects schemas from explicit policy first:

• use refund_intake
• use technical_support

When no policy-selected schema exists, the host can now fall back to saved factual premise state:

• damaged physical delivery premise -> refund_intake
• digital subscription login failure premise -> technical_support

Schema selection remains host-owned and happens before generateObject runs.

Why

The generic schema-selection examples now demonstrate both policy-driven and premise-driven schema selection.

This keeps the Vercel AI SDK example aligned with that same-domain story and shows the runtime path:

authoritative state -> selected schema -> generateObject request

without deriving schema selection from model output.

Example metadata

• Enforcement point: schema selection
• Domain: customer order/support intake
• Technology/runtime: Vercel AI SDK generateObject
• Example type: minimal example
• Does this use directive drafter? should normally be no: no
• Does this derive state from model output? must be no for primary examples: no

Checklist

• Repository scaffolding / governance change
• One primary enforcement point
• Explicit authoritative state
• No model-derived state mutation
• Adversarial stub or equivalent test
• Observable runtime behavior change
• Domain vocabulary is natural
• Framework is secondary to the enforcement point
• Docs updated
• Tests/smoke checks added where practical

Notes

Validation performed:

• cd typescript/examples/schema_selection/vercel_ai_sdk_generate_object && npm run build — passed
• cd typescript/examples/schema_selection/vercel_ai_sdk_generate_object && npm run typecheck — passed
• cd typescript/examples/schema_selection/vercel_ai_sdk_generate_object && npm test — passed, 15 passed, 1 skipped
• ./scripts/validate_typescript_fast.sh — passed
• npx --yes markdownlint-cli2 typescript/examples/schema_selection/vercel_ai_sdk_generate_object/README.md typescript/examples/schema_selection/README.md — passed
• cd typescript/examples/schema_selection/vercel_ai_sdk_generate_object && RUN_VERCEL_AI_SDK_LIVE_MODEL=1 OPENAI_API_KEY="$OPENAI_API_KEY" npm test — passed, 16 passed