fix(chat): correctly type thread and channel interface

[rayhanadev]

Why

The Thread and Channel interfaces in types.ts are missing toJSON() method declarations, even though the implementations (ThreadImpl, ChannelImpl) both implement it. This causes TypeScript errors when code typed against the public interfaces calls thread.toJSON() or channel.toJSON().

Closes #241

What changed?

• Added toJSON(): SerializedThread to the Thread interface
• Added toJSON(): SerializedChannel to the Channel interface
• Added type-only imports for SerializedThread and SerializedChannel in types.ts

Test plan

• pnpm validate passes (typecheck, lint, build, tests across all packages)
• thread.toJSON() now type-checks correctly against the Thread interface in sample code
• same w/ Channel

[vercel]

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[Copilot]

Pull request overview

This PR fixes a TypeScript typing gap in the chat SDK by aligning the public Thread and Channel interfaces with their concrete implementations, which already support serialization via toJSON().

Changes:

• Added toJSON(): SerializedChannel to the Channel interface.
• Added toJSON(): SerializedThread to the Thread interface.
• Added type-only imports for SerializedChannel and SerializedThread in types.ts.

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