Allow for deselection of all fixable errors when `check`ing

:wave:

Over in https://www.pantsbuild.org/ we're adding `ruff` support. Only one nit so far. We separate running tools that fix code from ones that only lint. Since `ruff` does both, the user will see `fix`able ruff errors when `lint`ing AND they'll see our "ruff made changes when trying to fix".

So ideally, in `lint` mode, we turn off the fixable errors. Not a dealbreaker, just a confusing message to our users.

The issue concerns Ruff's check/lint modes and the selection of fixable errors, but it names no files, tests, or entry points. Start by locating the command handling for check and the existing fixable-diagnostic filtering behavior, then identify the relevant tests. Done means lint mode can deselect all fixable errors without affecting non-fixable diagnostics or fix mode.

Written by the indexing model from the issue text.