Implement `onGetAvailableForms` Handler

[najuna-brian]

Problem

The onGetAvailableForms handler in FormulusMessageHandlers.ts was returning an empty array [], breaking core functionality for custom apps that depend on this API to discover and list available forms.

Impact

• Critical: Custom apps (WebView-based) cannot programmatically discover forms
• Custom apps cannot build form selection UIs dynamically
• Breaks the API contract defined in FormulusInterfaceDefinition.ts

Notes

• Native React Native screens (Forms, Observations) are unaffected (they use useForms() hook directly)
• This API is specifically for custom apps that need to discover forms programmatically