Expose thread.toJSON() on the public Thread type

[jacehwang]

Summary

• add toJSON(): SerializedThread to the public Thread interface
• add a consumer-facing type regression test for import type { Thread } from "chat"
• include a patch changeset for the public type fix

Problem

ThreadImpl already exposes toJSON() at runtime and the docs/examples use thread.toJSON(), but the public Thread type in dist/index.d.ts did not include that method. This caused consumer builds to fail with:

Property 'toJSON' does not exist on type 'Thread<...>'

Testing

• pnpm validate

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