[vertexai] Include appcheck token unconditionally (#6409)

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.

---------

Co-authored-by: rachelsaunders <[email protected]>

Author: rlazo

firebase-vertexai/CHANGELOG.md

@@ -1,4 +1,5 @@
 # Unreleased
+* [fixed] Fixed issue where Firebase App Check error tokens were unintentionally missing from the requests. (#6409)
 
 
 # 16.0.1
@@ -55,4 +56,3 @@
 * [feature] Added support for `responseMimeType` in `GenerationConfig`.
 * [changed] Renamed `GoogleGenerativeAIException` to `FirebaseVertexAIException`.
 * [changed] Updated the KDocs for various classes and functions.
-

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

@@ -99,9 +99,11 @@ internal constructor(
 
             if (token.error != null) {
               Log.w(TAG, "Error obtaining AppCheck token", token.error)
-            } else {
-              headers["X-Firebase-AppCheck"] = token.token
             }
+            // The Firebase App Check backend can differentiate between apps without App Check, 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) {