fix(core): Preserve OpenAI APIPromise methods like withResponse()

[Sai-Tom]

Summary

The OpenAI SDK returns APIPromise objects with additional methods like .withResponse(). The previous async/await instrumentation converted these to regular Promises, losing those methods.

This PR uses Proxy + handleCallbackErrors pattern (matching anthropic-ai) to preserve the original return type.

Changes

• Use Proxy instead of async function wrapper
• Use handleCallbackErrors for non-streaming responses (preserves Promise subclass)
• Add handleStreamingError helper for consistency
• Add tests for APIPromise.withResponse() preservation

Test Plan

• [x] yarn build:dev passes
• [x] yarn test passes
• [x] yarn lint passes
• [x] Added tests for APIPromise preservation