Describe the bug

When a streaming run is aborted via AbortSignal, result.state.usage returns all zeros (inputTokens: 0, outputTokens: 0, totalTokens: 0), even though the model has already generated partial content and consumed tokens.

This happens because:

1. The SDK accumulates usage only when a response_done event arrives (added in PR feat: track token usage while streaming responses for openai models #750)
2. When signal.abort() is called, the SDK catches the AbortError internally in #runStreamLoop and returns silently — the response_done event never arrives
3. The consumer's for await loop ends normally with no error, and result.state.usage remains at zero

This makes it impossible to track token consumption for cancelled runs, which is important for billing and quota management in production applications.

Debug information

• Agents SDK version: v0.4.6+
• Runtime environment: Node.js 22

Repro steps

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

async function main() {
  const controller = new AbortController();
  const agent = new Agent({ name: 'Test', instructions: 'You are a helpful assistant.' });

  const result = await run(agent, 'Write a long essay about AI', {
    stream: true,
    signal: controller.signal,
  });

  let content = '';
  for await (const event of result) {
    if (event.type === 'raw_model_stream_event') {
      const data = event.data as any;
      if (data.type === 'output_text_delta' && data.delta) {
        content += data.delta;
        if (content.length > 100) {
          controller.abort();
        }
      }
    }
  }

  console.log(`Content length: ${content.length}`); // > 0
  console.log(`Usage: ${JSON.stringify(result.state?.usage)}`); // All zeros!
}