Support parallel_tool_calls=False for ChatCompletion API (openai#513)

The current ChatCompletion API supports only `parallel_tool_calls=True` or `parallel_tool_calls=NOT_GIVEN` This PR is to support setting `parallel_tool_calls=False`, a common requirement in controlling agent tool use patterns (e.g. ensuring one tool call at the time, to facilitate desired tool calling sequence). I followed the merged [PR#333](openai#333) for consistency.

Author: ronaldchwu

src/agents/models/openai_chatcompletions.py

@@ -500,7 +500,11 @@ async def _fetch_response(
  span.span_data.input = converted_messages
 
  parallel_tool_calls = (
- True if model_settings.parallel_tool_calls and tools and len(tools) > 0 else NOT_GIVEN
+ True
+ if model_settings.parallel_tool_calls and tools and len(tools) > 0
+ else False
+ if model_settings.parallel_tool_calls is False
+ else NOT_GIVEN
  )
  tool_choice = _Converter.convert_tool_choice(model_settings.tool_choice)
  response_format = _Converter.convert_response_format(output_schema)