fix: allow returning a non-streaming reponse from onChatMessage()

[iTrooz]

Currently, doing

  async onChatMessage(
    onFinish: StreamTextOnFinishCallback<ToolSet>,
    _options?: { abortSignal?: AbortSignal }
  ) {
    return new Response("Hello !");
  }

from an AIChatAgent creates an error on the frontend, because doing so is only generating a text-delta event, and not the text-start and text-end events that should go in-between, according to https://ai-sdk.dev/docs/ai-sdk-ui/stream-protocol#text-start-part (from my understanding ?)

This PR fixes it by adding these 2 events.

[changeset-bot]

🦋 Changeset detected

Latest commit: 1e8d4f7

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@cloudflare/ai-chat	Patch
agents	Patch

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commit: 1e8d4f7