fix(chat): export standalone reviver for workflow-safe deserialization

[haydenbleasel]

Summary

• Adds a standalone reviver function exported from chat that deserializes Thread/Channel/Message objects without requiring a Chat instance (and its adapter Node.js dependencies)
• Updates Chat.reviver() to delegate to the standalone function internally
• Updates the durable chat sessions guide to use reviver from chat and dynamically import bot inside "use step" functions

Closes #243

Test plan

• pnpm validate passes (knip, lint, typecheck, tests, build)
• New tests for standalone reviver in serialization.test.ts
• Verify the updated guide pattern works with a real Vercel Workflow project

[vercel]

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Project	Deployment	Actions	Updated (UTC)
chat	Ready	Preview, Comment, Open in v0	Mar 16, 2026 8:10pm
chat-sdk-nextjs-chat	Ready	Preview, Comment, Open in v0	Mar 16, 2026 8:10pm

[bensabic]

Thanks for working through the Workflow sandbox issue, @haydenbleasel! The standalone reviver direction makes sense, but I don’t think this is quite ready to merge yet.

I found two follow-ups that still need to be addressed:

1. ChannelImpl has the same lazy round-trip bug as ThreadImpl. A chat:Channel revived via the new standalone reviver still can’t be serialized again without a registered singleton, because toJSON() derives adapterName from this.adapter.name instead of preserving the stored adapter name. This makes the new helper incomplete for channel payloads too.

2. The docs are now internally inconsistent around the new API. The durable session guide points readers to the Chat API for reviver, but apps/docs/content/docs/api/chat.mdx still only documents bot.reviver(), not the new root export.

If you fix the lazy serialization path by preserving adapterName on round-trip and update the API docs to cover the new export, this should be in a better shape.