Sets the default Chat object for dsprrr operations. This is stored in R options and persists for the session.
Examples
if (FALSE) { # \dontrun{
# Set a default OpenAI chat
set_default_chat(ellmer::chat_openai())
# Set a default Claude chat
set_default_chat(ellmer::chat_claude())
# Clear the default
set_default_chat(NULL)
} # }
