docs(adapters): correct Vercel AI route contract

[EmersonBraun]

Summary

• correct the canonical vercelAI guide to use the shipped route-based { api, headers?, retry? } configuration
• add a minimal Next.js streamText() route that returns UI Message Stream v1
• document the text-only boundary, plain-text fallback, authentication guidance, and the current tool-history limitation
• add an offline documentation contract test and an adapters patch changeset

Root cause

The provider page described an unsupported { model: LanguageModel } constructor even though the adapter has always called an existing HTTP route through { api }. This made the canonical example fail type checking and obscured the real incremental-migration boundary.

Impact

Users can keep their existing Vercel AI provider and route configuration while adding AgentsKit consumers. The guide now avoids claiming that AgentsKit tool-call history can be reconstructed after the adapter has reduced messages to role and string content.

Validation

• focused provider documentation contract: 1/1
• @agentskit/adapters tests: 509/509
• @agentskit/adapters TypeScript lint
• docs MDX lint: 418 files
• Doc Bridge gate
• git diff --check
• five-perspective review with no remaining finding at confidence 80 or higher
• repository pre-push quality gates and full production build passed

[github-actions]

Lighthouse

URL	Performance	Accessibility	Best practices	SEO
http://127.0.0.1:3000/	42	92	96	100
http://127.0.0.1:3000/docs	68	100	96	100
http://127.0.0.1:3000/docs/get-started/getting-started/quickstart	69	96	96	100
http://127.0.0.1:3000/stack	92	100	96	100
http://127.0.0.1:3000/showcase	89	100	96	100

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