[Luca Forstner (lforst)]

• Add explicit tracing helpers for AI SDK text batches, capturing prompts, outputs, token usage, and errors under one batch trace with a child span per request.

• Allow tracing to start after submission and finish in another process using saved context. Retrying completion updates the same spans.

• Keep submission, polling, and result collection in application code. The new helpers only capture trace data:

  import { startAISDKBatchTrace, completeAISDKBatchTrace } from "braintrust";
  
  // New: capture submitted requests and save context for later.
  const context = await startAISDKBatchTrace({ batch, requests });
  
  // New: finish tracing after collecting the batch results.
  await completeAISDKBatchTrace({ batch, requests, context, status, results });