Change the user's marketplace region (profile country).

This overwrites the country saved on the user's My Wine account, so the change applies across My Wine, not only to this conversation. It can be undone by calling this again with the previous country, which the result reports.

Only call this on the user's EXPLICIT request to change their region — never as part of the normal sommelier flow, and never on your own initiative. If unsure of the exact spelling, call list_available_regions first. If the named country isn't one the backend allows in a user profile, this refuses and does not change anything.

Args: country: Country name, as returned by list_available_regions (case-insensitive).