OpenAI 'file' message type is supported by Chat Completions

[krrishdholakia]

Code enforces files don't work -

openai-agents-python/src/agents/models/chatcmpl_converter.py

Line 242 in 901d2ac

raise UserError(f"File uploads are not supported for chat completions {c}")

But OpenAI docs + api do support this - https://platform.openai.com/docs/guides/pdf-files?api-mode=chat

[krrishdholakia]

cc: @pakrym-oai @rm-openai