fix: exclude iframes from focus-visible auto outline in Preflight (#20292)

## Description

Firefox's UA stylesheet already sets `iframe:focus-visible {
outline-style: none; }`, so Preflight's `:-moz-focusring { outline:
auto; }` rule overrides that and applies an unwanted auto outline to
focused iframes.

Adding `:where(:not(iframe))` to the selector preserves the improved
focus ring behavior for all other elements while respecting Firefox's
native iframe focus styling.

Fixes #19795

Edit by @RobinMalfait 

## Test plan

Before:
<img width="1054" height="383"
alt="file-f833142116d36e1e620290b97455f001"
src="https://github.com/user-attachments/assets/81aa170b-2cef-4964-934d-61f258335e1a"
/>


After:
<img width="1056" height="310"
alt="file-400d9214fedf5b43693e10580a4869de"
src="https://github.com/user-attachments/assets/8f8bb81d-6070-425d-8820-327bf9e88e79"
/>

---------

Co-authored-by: root <root@localhost.localdomain>
Co-authored-by: Kirk Loretz <kirk-loretz-fsn@users.noreply.github.com>
Co-authored-by: Robin Malfait <malfait.robin@gmail.com>

integrations/cli/index.test.ts

@@ -2903,7 +2903,7 @@ test(
           border-color: inherit;
           border-collapse: collapse;
         }
-        :-moz-focusring {
+        :-moz-focusring:where(:not(iframe)) {
           outline: auto;
         }
         progress {