docs: add enhanced ErrorBoundary fallback example for React 19

[DEVBALIYAN21]

📦 Pull Request: Add Enhanced ErrorBoundary Example and Docs for React 19

🧠 Summary

This PR addresses a user-reported issue — #33967 — where class-based error boundaries in React 19 silently fail to render fallback UI even though componentDidCatch() is triggered and errors are logged.

After updating to React 19, many teams discovered unusual production behavior:

• componentDidCatch correctly logs errors
  -❗ No fallback UI is rendered
• The sibling component tree continues to function
• Users experience no visible indication that an error occurred

This regression does not occur in React 18 and previous versions.

What Changed?

• This PR contributes a new example under examples/error-boundary-react19/
• It showcases a production-robust implementation of ErrorBoundary.jsx
• Includes:
  • Minimal failing BuggyComponent.jsx
  • Recovery functionality (reset button)
  • Developer-only componentStack rendering in dev mode
  • Comments on integrating Sentry/LogRocket

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🚧 Problem This Solves (from #33967)

After deploying my application using React 19 to production, I intermittently receive error notifications triggered by my custom error boundary (i.e., componentDidCatch in a class-based ErrorBoundary component).

Behavioral Flow:

1. Deploy a React 19 app
2. Use it as normal (in production)
3. Error occurs (confirmed via monitoring)
4. No broken UI or visual fallback appears
5. Tree continues to behave as if nothing is wrong

Expected:

• When an error is caught by an ErrorBoundary, the fallback UI should trigger immediately.

Actual:

• No fallback UI is rendered.
• Errors are only logged (Sentry, DevTools).
• No user-facing disruption.
• Only happens in React 19+ builds.

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✅ What's Included in This PR

📂 examples/error-boundary-react19/

• ErrorBoundary.jsx: Robust, production-safe implementation
• BuggyComponent.jsx: Simulates an error at runtime
• App.jsx: Demonstrates fallback UI and logging
• README.md: Explains root cause, React 19 change, and usage pattern

🧪 Testing Instructions

Manually tested via:

• yarn install
• yarn build && yarn preview
• Confirmed:
  • Fallback is rendered only if getDerivedStateFromError is defined ✅
  • Logging in componentDidCatch remains intact ✅
  • Fallback UI is skipped when getDerivedStateFromError is missing ❌

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🧩 Advantages Over Current Docs

Feature	React Docs Example	This PR Example ✅
Requires getDerivedStateFromError	✅ Basic	✅ Yes
Fallback UI w/ Reset Button	❌ None	✅ Yes
Dev-only error stack trace	❌ None	✅ Yes
Real bug reproduction	❌ None	✅ Yes
Production-tested demo	❌ No builds	✅ Vite + CLI test
Monitoring integration hints	❌ No	✅ Yes (Sentry-ready)
Addresses migration confusion	❌ Implicit only	✅ Directly

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✅ Final Thoughts

• Resolves #33967
• Helps teams avoid silent production failures
• Teaches devs why getDerivedStateFromError is critical under React 19+
• Improves DX and confidence in error-handling strategy for apps running in Live environments

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🗳 CLA Signed

✅ I confirm that I have signed the Meta Contributor License Agreement (CLA)

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