fix: #604 handle None choices in ChatCompletion response (#2850)

Co-authored-by: Kazuhiro Sera <seratch@openai.com>

Author: Spectual

src/agents/models/openai_chatcompletions.py

@@ -20,7 +20,7 @@
 
 from .. import _debug
 from ..agent_output import AgentOutputSchemaBase
-from ..exceptions import UserError
+from ..exceptions import ModelBehaviorError, UserError
 from ..handoffs import Handoff
 from ..items import ModelResponse, TResponseInputItem, TResponseStreamEvent
 from ..logger import logger
@@ -133,6 +133,15 @@ async def get_response(
                 stream=False,
                 prompt=prompt,
             )
+
+            if not response.choices:
+                provider_error = getattr(response, "error", None)
+                error_details = f": {provider_error}" if provider_error is not None else ""
+                raise ModelBehaviorError(
+                    f"ChatCompletion response has no choices (possible provider error payload)"
+                    f"{error_details}"
+                )
+
             message: ChatCompletionMessage | None = None
             first_choice: Choice | None = None
             if response.choices and len(response.choices) > 0: