I see no need to change move-beginning-of-line, because it already
behaves the same regardless of the length of the text after the prompt,
namely, moving to the start of the text instead of the start of the
prompt, and this is useful behavior.

This does, however, raise the question of whether invoking
move-end-of-line with point within the prompt string should just move to
the end of the prompt, which would make it more like the mirror image of
move-beginning-of-line in the minibuffer.  This is easy to do for the
case of the text after the prompt extending beyond window-width: