GitHub Actions workflow `discussion_answering.yml` fails on Q&A discussion creation with a Vertex AI 404 for `gemini-3.5-flash`

[ftnext]

Run: https://github.com/google/adk-python/actions/runs/27451592905
Discussion: #6103

Error:

google.genai.errors.ClientError: 404 NOT_FOUND.
Publisher Model `projects/***/locations/***/publishers/google/models/gemini-3.5-flash`
was not found or your project does not have access to it.

The failing step is Run Answering Script in ADK Answering Agent for Discussions. The workflow sets GOOGLE_GENAI_USE_VERTEXAI=1, and the model is hardcoded in:

• contributing/samples/adk_team/adk_answering_agent/agent.py
• contributing/samples/adk_team/adk_answering_agent/gemini_assistant/agent.py

This looks similar to #5696, but that issue covered triage workflows using Gemini Developer API. This failure is in the discussion answering workflow using Vertex AI.

Possible cause: GOOGLE_CLOUD_LOCATION may point to a region where gemini-3.5-flash is unavailable, or the Actions GCP project/service account does not have access to that model. Current docs list gemini-3.5-flash availability for global, us, and eu.