[messaging] Enhance Expo notification icon configuration support and documentation

[Copilot]

Summary

This PR addresses issue #8616 by significantly improving the documentation and developer experience for the existing Expo notification icon configuration feature in @react-native-firebase/messaging.

The messaging package already includes an Expo config plugin (added in v18.7.0 via #7369) that automatically configures Firebase notification icons and colors from app.json, but this functionality was poorly documented and largely unknown to users.

Changes Made

📚 Enhanced Documentation

• Updated docs/messaging/usage/index.md with comprehensive Expo configuration section including:
  • Step-by-step setup instructions
  • Example app.json configuration
  • Android vs iOS behavior explanation
  • Notification icon requirements (pure white PNG)
• Updated packages/messaging/README.md with clear Expo setup instructions and example
• Added packages/messaging/EXPO_EXAMPLE.md with detailed examples and best practices

🔧 Improved Developer Experience

• Enhanced warning messages with actionable configuration examples when notification config is missing
• Added success logging when configuration is applied successfully
• Better error messaging clearly branded with [@react-native-firebase/messaging]

✅ Enhanced Testing

• Added comprehensive test coverage for various configuration scenarios
• Tests verify proper warning behavior when configuration is missing
• Tests verify successful configuration and logging messages

How It Works

The existing Expo plugin automatically:

1. Reads expo.notification.icon and expo.notification.color from app.json
2. Configures Android manifest with proper Firebase messaging metadata:
   • com.google.firebase.messaging.default_notification_icon
   • com.google.firebase.messaging.default_notification_color
3. Provides helpful warnings with examples when configuration is missing
4. Logs success messages when configuration is applied

Example Configuration

{
  "expo": {
    "notification": {
      "icon": "./assets/notification-icon.png",
      "color": "#FF0000"
    },
    "plugins": [
      "@react-native-firebase/messaging"
    ]
  }
}

Before vs After

Before: Users were unaware this feature existed and requested it as a new feature
After: Clear documentation and improved messaging make the feature discoverable and easy to use

Testing

• ✅ All existing tests pass
• ✅ New test coverage for edge cases and validation
• ✅ Linting and code quality checks pass
• ✅ Plugin builds successfully

This PR transforms an existing but hidden feature into a well-documented, user-friendly solution that fully addresses the feature request in #8616.

Warning

Firewall rules blocked me from connecting to one or more addresses

I tried to connect to the following addresses, but was blocked by firewall rules:

• docs.expo.dev
  • Triggering command: curl -s REDACTED (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to my firewall allow list

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Name	Status	Preview	Comments	Updated (UTC)
react-native-firebase	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jul 17, 2025 0:54am

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[Ehesp]

(Playing around, ignore)