test(components): backfill advanced surface coverage

[bntvllnt]

Related to #231

Summary

• Add MDXContent coverage for markdown rendering, injected component import stripping, and markdown fallback after MDX evaluation errors.
• Add ModelSelector coverage for selected model state, provider metadata search, and selection callbacks.
• Add SocialFAB coverage for hidden state, desktop hover tracking, mobile backdrop close, and share platform launch.
• Add Slideshow coverage for portal scroll locking, completion gating, TOC navigation, and keyboard shortcuts.

Validation

• pnpm -F @vllnt/ui exec vitest run src/components/mdx-content/mdx-content.test.tsx src/components/model-selector/model-selector.test.tsx src/components/social-fab/social-fab.test.tsx src/components/slideshow/slideshow.test.tsx (4 files, 14 tests)
• pnpm -F @vllnt/ui exec eslint src/components/mdx-content/mdx-content.test.tsx src/components/model-selector/model-selector.test.tsx src/components/social-fab/social-fab.test.tsx src/components/slideshow/slideshow.test.tsx
• pnpm -F @vllnt/ui lint
• pnpm -F @vllnt/ui exec tsc --noEmit --project tsconfig.build.json
• pnpm build
• pnpm test:once (220 files, 1229 tests)
• pnpm -F @vllnt/ui check:use-client
• pnpm -F @vllnt/ui check:stories
• pnpm -F @vllnt/ui storybook:verify
• pnpm -F @vllnt/ui build-storybook
• git diff --check
• git diff --cached --check

CI

• PASS on PR test(components): backfill advanced surface coverage #353 as of 2026-05-13 06:16 UTC: Quality Gates, Storybook Tests, Visual Regression, Build Storybook, Verify Stories, PR Issue Link, react-doctor, and Vercel previews.

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Project	Deployment	Actions	Updated (UTC)
storybook	Ready	Preview, Comment	May 13, 2026 6:13am
ui.vllnt.ai	Ready	Preview, Comment	May 13, 2026 6:13am