[vertexai] Include appcheck token unconditionally

The app check backend can differentiate between apps that do not use
appcheck at all, and those sending invalid data, by using a special
placeholder token when errors happen. We were previously not including
the token value in error cases, but we should.

Author: rlazo

firebase-vertexai/src/main/kotlin/com/google/firebase/vertexai/GenerativeModel.kt

@@ -99,9 +99,10 @@ internal constructor(
 
             if (token.error != null) {
               Log.w(TAG, "Error obtaining AppCheck token", token.error)
-            } else {
-              headers["X-Firebase-AppCheck"] = token.token
             }
+            // The appcheck backend can differentiate between apps without appcheck, and wrongly
+            // configured apps by verifying the value of the token, so it always needs to be included.
+            headers["X-Firebase-AppCheck"] = token.token
           }
 
           if (internalAuthProvider == null) {