Merge https://github.com/google/adk-python/pull/6113
## Summary
- Configure the discussion answering agents through `LLM_MODEL_NAME` instead of
hardcoding `gemini-3.5-flash`.
- Let the GitHub workflow override both the answering model and Vertex location
with repository variables.
- Downgrade the default model from `gemini-3.5-flash` to `gemini-2.5-flash` because
`gemini-3.5-flash` 404s on Vertex AI Search (#6104).
- Document the new knobs.
Fixes#6104
## Testing
- `git diff --check`
- `python3.12 -m py_compile contributing/samples/adk_team/adk_answering_agent/settings.py contributing/samples/adk_team/adk_answering_agent/agent.py contributing/samples/adk_team/adk_answering_agent/gemini_assistant/agent.py`
Co-authored-by: Kathy Wu <wukathy@google.com>
COPYBARA_INTEGRATE_REVIEW=https://github.com/google/adk-python/pull/6113 from Haihan-Jiang:codex/adk-python-answering-model-config aea24eab14eaf479b7d42f59f25a94e0baa713a5
PiperOrigin-RevId: 947224603
Haihan Jiang
de4ac2d93f