fix: #638 add unit tests demonstrating how to get usage in streaming mode

[seratch]

Developers can access the usage data for each response:

import { Agent, run } from '@openai/agents';

async function main() {
  const agent = new Agent({
    name: 'Joker',
    instructions: 'You are a helpful assistant.',
  });
  const stream = await run(agent, 'Please tell me 5 jokes.', { stream: true });
  await stream.completed;
  for (const response of stream.rawResponses) {
    console.log(response.usage);
  }
}

main().catch((error) => {
  console.error(error);
  process.exit(1);
});

This pull request adds new unit test patterns verifying the behavior. Fixing #638

[changeset-bot]

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Latest commit: 5b710e9

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