Align baggageScope.dispose() pattern with Devin sample

[Copilot]

Addresses feedback on inconsistent baggageScope.dispose() pattern in the OpenAI Agent sample. The Devin sample calls dispose() after run() completes (not in a finally block), while this sample was wrapping the entire operation in try-finally.

Changes

• Removed try-finally wrapper around baggageScope.run() in nodejs/openai/sample-agent/src/agent.ts
• Call dispose() after run() completes, matching the Devin sample pattern

// Before
try {
  await baggageScope.run(async () => { ... });
} finally {
  baggageScope.dispose();
}

// After
await baggageScope.run(async () => { ... });
baggageScope.dispose();

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