gptel-transient: Use width of the full frame instead of window

[pabl0]

• gptel-transinent.el: (gptel--read-with-prefix, gptel-system-prompt--format, gptel--setup-directive-menu, gptel-tools): Use frame-width' instead of window-width' when truncating things when using the minibuffer. Otherwise they get truncated too early when the user has horizontally split windows.