fix: add missing AppAttestProviderFactory implementation

[Njones27]

Summary

Fixes the release build failure after PR #63. The Firebase iOS SDK provides AppAttestProvider but does not ship a factory class for it — only AppCheckDebugProviderFactory is built in. The release-build call to AppAttestProviderFactory() failed to compile with:

Cannot find 'AppAttestProviderFactory' in scope

This PR adds a small custom factory conforming to AppCheckProviderFactory that returns an AppAttestProvider, matching the pattern Firebase recommends for iOS 14+ App Attest integration.

Change

A 7-line addition to WalkWorthyApp.swift:

final class AppAttestProviderFactory: NSObject, AppCheckProviderFactory {
    func createProvider(with app: FirebaseApp) -> AppCheckProvider? {
        return AppAttestProvider(app: app)
    }
}

Test plan

• Build Archive succeeds in Xcode Cloud / local archive
• Debug build still works (uses AppCheckDebugProviderFactory)
• Release build uses App Attest

🤖 Generated with Claude Code

[Njones27]

hot fix, we push without checks because I'm awesome