Make LinkNewDeviceScreen fall back to the App Lock PIN code when there's no passcode. (#5794)

* Fall back to the AppLock PIN code when there's no passcode.

* Add extra tests on the LinkNewDeviceFlowCoordinator.

* Misc tweaks for clarification.

* Regenerate snapshots.

UnitTests/Sources/AppLock/AppLockScreenViewModelTests.swift

@@ -43,6 +43,19 @@ final class AppLockScreenViewModelTests {
         #expect(result == .appUnlocked)
     }
     
+    @Test
+    func cancelVerifyDeviceOwner() async throws {
+        // Given a screen shown to verify the device owner.
+        viewModel = AppLockScreenViewModel(appLockService: appLockService, mode: .verifyDeviceOwner)
+        
+        // When the user cancels.
+        let deferred = deferFulfillment(viewModel.actions) { $0 == .cancelVerifyDeviceOwner }
+        viewModel.context.send(viewAction: .cancelVerifyDeviceOwner)
+        
+        // Then the cancellation should be reported.
+        try await deferred.fulfill()
+    }
+    
     @Test
     func forgotPIN() async throws {
         // Given a fresh launch of the app.