🔧 chore(llms.py): add format_openai_field method to format OpenAI field display names

The `format_openai_field` method formats the display name of fields that contain the string "openai" in their name. It capitalizes the first letter of "OpenAI", replaces underscores with spaces, and replaces "Api" with "API". This improves the readability and consistency of the display names of OpenAI fields.