fix(a11y): stop decorative canvas graphics reporting as unnamed (#14660)

IBM Equal Access flagged the flow canvas with svg_graphics_labelled
(WCAG 1.1.1) once per edge plus once for the dot grid: ReactFlow renders
a bare <svg> wrapper around every edge and a bare <svg> for the grid, and
an unnamed <svg> is an unnamed graphic.

The grid is wrapped in an aria-hidden container. The edge wrappers only
get role="presentation" -- the named, tabbable <g role="group"> that
carries the edge's accessible name lives inside them, so aria-hidden
there would erase every edge name. ReactFlow exposes no prop hook for
either element, so the wrappers are marked by a MutationObserver hook.