feat: add live-model validation for Vercel schema selection

[rlippmann]

Summary

Add an opt-in live-model validation path for the TypeScript Vercel AI SDK schema-selection example.

The provider-free tests remain the canonical proof. The new live-model path demonstrates that authoritative Context Compiler state continues to control schema selection when using a real Vercel AI SDK generateObject call.

Why

The repository already includes a provider-free Vercel AI SDK schema-selection example.

This change adds a Tier 3 validation path showing that the same host-controlled schema-selection behavior survives a real model call while keeping provider-free tests as the primary proof.

Example metadata

• Enforcement point: schema selection
• Domain: refund intake / technical support
• Technology/runtime: Vercel AI SDK (ai, @ai-sdk/openai)
• Example type: reference integration
• 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:

• npm run build
• npm run typecheck
• npm test
• ./scripts/validate_typescript_fast.sh
• markdownlint

Live-model validation:

• gpt-4o-mini ✅
• gpt-5-mini ✅
• gpt-5 ✅
• gpt-5.4 ✅

The live-model path is opt-in and does not change schema-selection semantics.