Sending FCM notification with Fids Return NotRegistered

### Environment

- FirebaseAdmin: 3.6.0
- .NET: (your version)
- Platform: iOS
- FCM HTTP v1 (through Firebase Admin SDK)

### Description

I'm trying to migrate from registration tokens to FIDs as recommended by the SDK.

When sending a notification to an Apple device using the `Fid` property of `Message`, the Admin SDK always returns a `FirebaseMessagingException` with the error `NotRegistered`.

However, using **the exact same FID** from the Firebase Console ("Send test message"), the notification is delivered successfully to the device.

This suggests either:
- the .NET Admin SDK is not sending the request correctly when using `Message.Fid`, or
- there is a backend/API discrepancy between the Admin SDK and the Firebase Console.

### Code

```csharp
var message = new Message
{
Fid = fid,
Notification = new Notification
{
Title = "Test",
Body = "Hello"
}
};

await FirebaseMessaging.DefaultInstance.SendAsync(message);