openai: Don't append tool calls to prior assistant messages (#29969)

Closes https://github.com/zed-industries/zed/issues/29821

Release Notes:

- Fixed an issue in the agent panel where OpenAI requests would fail if
the assistant begins its response with a tool call.
2eb10ab9fbMax Brunsfeld committed on 5/6/2025, 5:04:56 AM· committed by GitHubparent55fd835
1 file changedLine totals unavailable