LLMs generally but more so small models will keep going and generate seemingly unrelated things. On the frontend tools like Copilot and Ghostwriter do a lot of things like use stopwords or simply not show completions outside a single block.
As for your prompt, it's following your prompt a little too closely and generating just the function. You can however condition it that this is the start of the program it will do the import, e.g.
# python function that returns a random integer between min and max
import