Spinning this out of some of the discussion on #12595, as requested by @zanieb.

That PR adds support for displaying fixes, but hard codes in displaying only fixes at the Applicability::Safe level in the Display implementation. That really isn’t where it belongs: the TextEmitter is more appropriate. However, that also bumps into a number of other pieces related to design questions around what exactly to do for displaying those.

The Diff itself should simply produce the diff and any appropriate CTA, without configuration or handling for availability. The TextEmitter should be responsible for determining what to print. However, at present that is somewhat coupled because the Diff::from_message consumes the Message from which it is constructed, and produces an Option<Diff>. (As an aside: an implementation like from_message should idiomatically use std::from::From.)

Additionally, this should make it possible to solve the other problem with the current implementation: that it hard-codes displaying only safe fixes even when the user runs ruff check --unsafe-fixes (which does work in the absence of --fix, and should only display) fixes.