Answer on a plain OpenAI key from the AG2 Bot

[asasemahmed]

What this changes

Picked with a plain OpenAI key, the AG2 Bot fails every run. Compose passes the picked harness
OPENAI_BASE_URL: ${OPENAI_CONTAINER_BASE_URL:-${OPENAI_BASE_URL:-}}, and for a plain OpenAI key
that is empty. OpenAIConfig(model=model) leaves base_url as None, so the OpenAI SDK reads
OPENAI_BASE_URL itself, and it falls back to https://api.openai.com/v1 only when that is absent.
Given "", the request has no host.

The Anthropic branch just above already handles exactly this: "Compose exports missing overrides as
""; the SDK only defaults an absent URL." The OpenAI branch now does the same, passing
base_url=(OPENAI_BASE_URL or "").strip() or "https://api.openai.com/v1". An OpenAI-compatible
endpoint is passed through as before.

#649 edits the provider and model lines of this file; this change is a separate hunk below them.

Where it runs

• New state that outlives a request? None. The config is chosen once at import.
• What happens on the second replica? The same, from the same environment.
• Anything serialised? No.
• Anything fanned out to a browser? No.
• New listener, port, or schedule? No.

Boundary and audit

• Every acting call still goes through the gateway: nothing on the server changes, and the
  harness still refuses a request without the server's token.
• New refusals and new failures each write a row: none added.
• Nothing new is trusted from the client.

Changelog

• An entry in CHANGELOG.md under Unreleased.

Proof

• New test_an_openai_key_uses_the_official_endpoint_when_compose_sets_a_blank_url, the twin of
  the Anthropic test above it: the real AG2 and OpenAI clients, with only the httpx2 transport
  replaced and routed into this file's fake provider. It asserts the request went to
  https://api.openai.com/v1/chat/completions with Bearer test-key, and a finished run.
• Python 3.12, the CI step's commands, run in a container with no network: this branch
  6 passed. With src from main: 1 failed, 5 passed, the new case, whose request goes to a
  relative /chat/completions.
• Versions pip resolved: ag2 1.1.0, openai 3.19.2, anthropic 1.8.0, httpx2 2.13.1.
• Not run: a real OpenAI key, and the published image.