Remove unnecessary OpenAI key setup from agents.py
The OpenAI Agents SDK automatically reads OPENAI_API_KEY from the
environment on import, making the explicit set_default_openai_key()
call redundant. This removes provider-specific logic from the routing
layer, keeping agents.py provider-agnostic.
- Remove 6-line OpenAI key setup block from main_async()
- Remove related test patches from test_agents_ndjson.py
- Remove stub from agents_stub.py
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>