[v2] chore(examples): render every React example in StrictMode

[dasjideepak]

🎯 Changes

Part of #2305. Per the discussion there, every example should run in StrictMode (ports stay as they are).

• Wrapped <App /> in React.StrictMode in the 6 Vite examples that rendered without it: array, basic, basic-splitting-form, field-groups, ui-integration-dnd-kit and ui-integration-shadcn. Same pattern as simple, large-form and the others that already had it.
• react/nextjs: set reactStrictMode: true in next.config.ts, matching next-server-actions and next-server-actions-zod.
• react/expo: index.ts now registers a small Root component that renders App inside StrictMode. It uses createElement because the entry file is .ts (it's the package's main).

Already in StrictMode, so unchanged: compiler, large-form, multi-step-wizard, query-integration, simple, standard-schema, and tanstack-start (Start's default client entry hydrates inside <StrictMode>).

The other frameworks don't have a StrictMode equivalent, so they're untouched.

I ran tsc and vite build for each changed Vite example, tsc for nextjs and expo, and prettier on the changed files.

✅ Checklist

• I have followed the steps in the Contributing guide.
• I have tested code changes locally with pnpm test:pr, or these tests do not apply to this pull request. (Examples only; I type-checked and built each changed example.)
• I fully understand the code in this pull request, including any code generated with AI assistance.

🚀 Release Impact

• This change affects published code, and I have generated a changeset.
• This change is docs/CI/dev-only (no release).

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📥 Commits

Reviewing files that changed from the base of the PR and between 541abc8 and 13215d7.

📒 Files selected for processing (8)

• examples/react/array/src/index.tsx
• examples/react/basic-splitting-form/src/index.tsx
• examples/react/basic/src/index.tsx
• examples/react/expo/index.ts
• examples/react/field-groups/src/index.tsx
• examples/react/nextjs/next.config.ts
• examples/react/ui-integration-dnd-kit/src/index.tsx
• examples/react/ui-integration-shadcn/src/index.tsx

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[dasjideepak]

Moved into #2406 to keep the series smaller.