fix(appcheck,macos): fix how appAttestWithDeviceCheckFallback is working on fallback

[Lyokone]

Description

AppleProvider.appAttestWithDeviceCheckFallback chose its provider on OS version alone, so on macOS 14+ it always installed AppAttestProvider and never fell back to DeviceCheck. Since App Attest also requires device support — commonly absent on macOS — every token request then failed with "The attestation provider AppAttestProvider is not supported on current platform and OS version", raised by the SDK at getToken time rather than at activation. AppAttestProvider(app:) is declared nullable but never returns nil in the current SDK, so nothing surfaced the problem earlier either.

The fix checks DCAppAttestService.shared.isSupported — the same capability the SDK checks — before selecting App Attest, and falls back to DeviceCheckProvider otherwise:

var appAttestProvider: (any AppCheckProvider)?
if #available(iOS 14.0, macOS 14.0, *), DCAppAttestService.shared.isSupported {
  appAttestProvider = AppAttestProvider(app: app)
}
delegateProvider = appAttestProvider ?? DeviceCheckProvider(app: app)

The ?? also covers the nullable initializer, so the fallback holds if a future SDK version starts returning nil. iOS behaviour is unchanged where App Attest is supported. Callers can drop Platform.isMacOS special-casing after this.

Not addressed here: DeviceCheck being unreliable on some macOS versions is upstream (firebase-ios-sdk#10095, still open). Separately, the appAttest case immediately above silently substitutes AppCheckDebugProvider when the OS is too old — shipping a debug provider in a release build seems worse than a clear error, but that is a behaviour change and is left out of this PR deliberately.

Related Issues

• Fixes [firebase_app_check]: MacOS : The attestation provider AppAttestProvider is not supported on current platform and OS version #17057

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
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• I read and followed the Flutter Style Guide.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

• Yes, this is a breaking change.
• No, this is not a breaking change.

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