test(webview): finish CSS transitions after the visual theme changes

applyVisualTheme changes the theme classes after the story mounts. The
Button transition-colors class then changes the border color over about
150 ms. expectContrast can read a color between the two themes, so the
WelcomeViewProvider contrast check fails at random.

Finish each running CSSTransition after the theme classes change. Other
animation types stay as they are, because finishing an infinite CSS
animation throws an error.

Fixes #2326

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: rrewll

webview-ui/playwright/themes.ts

@@ -24,5 +24,11 @@ export async function applyVisualTheme(page: Page, theme: VisualTheme) {
 		document.body.className = bodyClass
 		document.body.removeAttribute("style")
 		document.body.dataset.vscodeThemeId = themeId
+
+		for (const animation of document.getAnimations()) {
+			if (animation instanceof CSSTransition) {
+				animation.finish()
+			}
+		}
 	}, theme)
 }