Add tests for linking a new device. (#4934)

* Replace GrantLoginWithQrCodeHandlerSDKMock with LinkNewDeviceServiceMock.

Add tests for all initial states on the QRCodeLoginScreen.

* Add tests for linking both mobile and desktop devices.

* Add UI tests for linking a new device.

* Don't show the Link Desktop Computer button when running on macOS.

This mirrors the decision to hide the Sign In With QR Code button on the start screen.

ElementX/Sources/Mocks/LinkNewDeviceServiceMock.swift

@@ -0,0 +1,33 @@
+//
+// Copyright 2025 Element Creations Ltd.
+//
+// SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial.
+// Please see LICENSE files in the repository root for full details.
+//
+
+import SwiftUI
+
+extension LinkNewDeviceServiceMock {
+    static var mockQRCodeImage: UIImage {
+        mockBase64QRCode.data(using: .utf8).flatMap { UIImage(qrCodeData: $0) } ?? UIImage()
+    }
+    
+    static let mockBase64QRCode = """
+    TUFUUklYAgS0yzZ1QVpQ1jlnoxWX3d5jrWRFfELxjS2gN7pz9y+3PABaaHR0
+    cHM6Ly9zeW5hcHNlLW9pZGMubGFiLmVsZW1lbnQuZGV2L19zeW5hcHNlL2Ns
+    aWVudC9yZW5kZXp2b3VzLzAxSFg5SzAwUTFINktQRDQ3RUc0RzFUM1hHACVo
+    dHRwczovL3N5bmFwc2Utb2lkYy5sYWIuZWxlbWVudC5kZXYv
+    """
+    
+    struct Configuration {
+        var linkMobileProgressPublisher: LinkNewDeviceService.LinkMobileProgressPublisher = .init(.starting)
+        var linkDesktopProgressPublisher: LinkNewDeviceService.LinkDesktopProgressPublisher = .init(.starting)
+    }
+    
+    convenience init(_ configuration: Configuration) {
+        self.init()
+        
+        linkMobileDeviceReturnValue = configuration.linkMobileProgressPublisher
+        linkDesktopDeviceWithReturnValue = configuration.linkDesktopProgressPublisher
+    }
+}