feat(chat): allow disabling fallback streaming placeholder

[vincenzodomina]

Summary

• Add fallbackStreamingPlaceholderText?: string | null to ChatConfig.
• Preserve null through Chat and ThreadImpl so fallback streaming can skip the initial "..." post.
• Keep the default behavior unchanged: omitting the option still posts "..." immediately.

Motivation

Issue #140 only needs a way to suppress the initial placeholder on post+edit fallback adapters (for example Telegram), so this PR keeps the change scoped to that behavior.

When the placeholder is null, the first real streamed chunk creates the message and the existing edit loop continues from there.

See issue: #140

Test plan

• pnpm --dir packages/chat exec vitest run src/thread.test.ts src/chat.test.ts
• pnpm --dir packages/chat run typecheck

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[haydenbleasel]

Thanks for the contribution @vincenzodomina! 🙌

I pushed a few small fixes on top of your work:

• Added a missing test for the empty stream + placeholder=null edge case
• Replaced the (customChat as any).createThread test with one that exercises the full Chat → Thread pipeline through public APIs (less fragile)
• Sorted interface members alphabetically to pass the Ultracite linter
• Added a changeset
• Updated the docs

Full validation passes — should be good to go!

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1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
chat	Ignored	Open in v0	Mar 1, 2026 3:14am

[vincenzodomina]

Whoa thanks for the fast merge!