[vertexai] Include appcheck token unconditionally

[rlazo]

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.

[github-actions]

Release note changes

The following release notes were modified. Please ensure they look correct.

Release Notes

firebase-vertexai

### {{firebase_vertexai}} version 16.0.2 {: #vertex-ai_v16-0-2}

* {{fixed}} Fixed issue where Firebase App Check error tokens were unintentionally missing from the requests. GitHub [#6409](//github.com/firebase/firebase-android-sdk/issues/6409){: .external}

[github-actions]

Vertex AI Mock Responses Check ⚠️

A newer major version of the mock responses for Vertex AI unit tests is available. update_responses.sh should be updated to clone the latest version of the responses: v5.1

[github-actions]

Test Results

 20 files   -  1 002   20 suites   - 1 002   15s ⏱️ - 36m 12s
112 tests  -  5 697  112 ✅  -  5 675  0 💤  - 22  0 ❌ ±0 
224 runs   - 11 479  224 ✅  - 11 435  0 💤  - 44  0 ❌ ±0 

Results for commit 09c3ef9. ± Comparison against base commit d96507b.

This pull request removes 5697 tests.

com.google.android.datatransport.cct.CctBackendFactoryTest ‑ create_returnCCTBackend_WhenBackendNameIsCCT
com.google.android.datatransport.cct.CctDestinationTest ‑ cctDestination_shouldOnlySupportProtoAndJson
com.google.android.datatransport.cct.CctDestinationTest ‑ cctDestination_shouldSupportProtoAndJson
com.google.android.datatransport.cct.CctTransportBackendTest ‑ decorate_whenOffline_shouldProperlyPopulateNetworkInfo
com.google.android.datatransport.cct.CctTransportBackendTest ‑ decorate_whenOnline_shouldProperlyPopulateNetworkInfo
com.google.android.datatransport.cct.CctTransportBackendTest ‑ schedule_shouldAddCookieOnPseudonymousIds
com.google.android.datatransport.cct.CctTransportBackendTest ‑ schedule_shouldDropCookieOnMixedPseudonymousIds
com.google.android.datatransport.cct.CctTransportBackendTest ‑ send_CompressedResponseIsUncompressed
com.google.android.datatransport.cct.CctTransportBackendTest ‑ send_whenBackendRedirectsMoreThan5Times_shouldOnlyRedirect4Times
com.google.android.datatransport.cct.CctTransportBackendTest ‑ send_whenBackendRedirects_shouldCorrectlyFollowTheRedirectViaPost
…

♻️ This comment has been updated with latest results.