Support OPENAI_BASE_URL in tracing

[tabletcorry]

Agent tracing was hardcoded against the API endpoint, so any client using a proxy or alternate route to the OpenAI AI endpoint would encounter errors.

So this modify the tracing init code to use OPENAI_BASE_URL when present, if the local environment hasn't overridden anything.

Note: Because this code path is generally executed at import time, the environment variable needs to be set prior to importing any openai-agents code.