fix(mlc): ignore non-text chat deltas before streaming

[chux0519]

1. Issue: In the iOS MLC demo, after finishing “Stream Text” the status panel throws TypeError: Cannot read property
   'length' of undefined.
2. Root cause: The native bridge’s last onChatUpdate emits {"delta": {"role": "assistant"}}. The JS layer forwards
   delta.content to the Vercel AI SDK without type checks, so the SDK receives undefined and crashes when it reads length.
3. Fix: Update onChatUpdate in packages/mlc/src/ai-sdk.ts to enqueue a text-delta only when delta.content is a string;
   ignore any other shape.

---

Note

Add a type guard in packages/mlc/src/ai-sdk.ts to only enqueue text-delta when delta.content is a string, ignoring other update shapes.

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