#34494 base red: Settings' progress fill is not a surface; the model menu's + draws at 12px

Fixes two webview base reds that every feat-cmux-next push run since #33176 / #32980 shows (for example 41742234192 at 842ce345 and 41745017210 at 4165f6f0):

• webviews/src/pages/settings/render.test.tsx "no rule declares a background other than transparent or the two interaction tokens": regressed by #33176, which added .app-update-progress::-webkit-progress-value and ::-moz-progress-bar with background: var(--text). A progress bar's value fill is not a surface, so the test now exempts only those two progress-value pseudo-elements at that value. Containers stay checked, and the neutral bar #33176 shipped stays as it is.
• webviews/test/check-icon-scale.test.ts: regressed by #32980. The model menu's PlusIcon (QuickModelColumns.tsx) drew at 11px, off the scale. #32980 landed about an hour after the check itself (#33595). The icon now draws at 12px, which is on the scale, matches its 12 viewBox and matches the CheckIcon beside it. The baseline only shrinks, so raising it isn't allowed.

Model picker surface: design/model-picker.md says nothing about icon size; this changes the + by 1px.

Agent session: agent session

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Summary by cubic

Fixes two webview base reds by updating the settings render test and the model menu's PlusIcon size.

• The render test now exempts progress-bar value fills (::progress-value pseudo-elements) from the surface check, since a progress bar's filled part is its value, not a surface.
• The model menu's PlusIcon now draws at 12px instead of 11px, matching its viewBox and the CheckIcon beside it, and putting it back on the icon scale.