feat(chat): add streaming options to thread.post()

[dancer]

summary

adds StreamingPlan PostableObject for passing platform-specific streaming options through thread.post() instead of bypassing the Thread abstraction via adapter.stream() directly

const stream = new StreamingPlan(result.fullStream, {
  groupTasks: "plan",
  endWith: [feedbackBlock],
});
await thread.post(stream);

• groupTasks controls task display mode ("plan" or "timeline")
• endWith attaches Block Kit elements when the stream stops
• updateIntervalMs controls fallback post+edit cadence
• recipientUserId/recipientTeamId auto-populated from message context
• adapters without native streaming fall back to post+edit automatically

test plan

• ran pnpm --filter chat test - all tests pass
• tested StreamingPlan with full options, groupTasks only, and endWith only
• verified post() return type matches the PostableObject overload contract

[vercel]

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Project	Deployment	Actions	Updated (UTC)
chat	Ready	Preview, Comment, Open in v0	Apr 15, 2026 1:31am
chat-sdk-nextjs-chat	Ready	Preview, Comment, Open in v0	Apr 15, 2026 1:31am

[vercel]

Additional Suggestion:

When a StreamMessage is passed to thread.post(), TypeScript infers the return type as Promise<StreamMessage> via the generic post<T extends PostableObject>(message: T): Promise<T> overload, but the runtime actually returns a SentMessage, causing type unsafety.