Fix RenderFlex overflow in debugging controls at narrow widths (#9949)

* Fix RenderFlex overflow in debugging controls at narrow widths

The debugging controls drop their button labels below
DebuggingControls.minWidth, but the remaining icon-only content still
does not fit below roughly 630px, so the controls Row overflowed at
widths that are realistic for DevTools embedded in an IDE side panel.

Make the controls scroll horizontally so every control stays reachable
instead of being clipped behind an overflow error. The file explorer
button stays pinned outside the scroll view so it does not scroll out of
reach, which keeps the wide layout visually unchanged.

Fixes #4917

* Add release note for the debugging controls overflow fix

* Point the release note at the actual PR number

* Move test setup into a setUp block