feat: OpenAI Agent Stream (#597)

.changeset/khaki-rivers-unite.md

examples/agent/stream_openai_agent.ts

packages/core/src/agent/openai/worker.ts

packages/core/src/agent/runner/base.ts

packages/core/src/agent/runner/types.ts

packages/core/src/agent/types.ts

packages/core/src/engines/chat/types.ts