[Firebase AI] Add integration placeholder for debug token env var (#15528)

Author: andrewheard

FirebaseAI/Tests/TestApp/FirebaseAITestApp.xcodeproj/xcshareddata/xcschemes/FirebaseAITestApp-SPM.xcscheme

@@ -66,10 +66,15 @@
       <CommandLineArguments>
          <CommandLineArgument
             argument = "-FIRDebugEnabled"
-            isEnabled = "YES">
+            isEnabled = "NO">
          </CommandLineArgument>
       </CommandLineArguments>
       <EnvironmentVariables>
+         <EnvironmentVariable
+            key = "FIRAAppCheckDebugToken"
+            value = "Configure an App Check debug token from https://console.firebase.google.com/project/_/appcheck/apps"
+            isEnabled = "NO">
+         </EnvironmentVariable>
          <EnvironmentVariable
             key = "FIRGCloudAuthAccessToken"
             value = "Run `gcloud auth print-access-token` to obtain as access token for direct Vertex AI testing."