Classic Duo Chat results in model switching default namespace missing G3002 error in 6.40.0 and later

Checklist

• I'm using the latest version of the extension (see the latest version in the right column of this page)
  • Extension version: 6.40.1
• I'm using the latest VS Code version (find the latest version here)
  • VS Code version: 1.103.2
• I'm using a supported version of GitLab (see README for the supported version)
  • GitLab version: happens on gitlab.com

Summary

Steps to reproduce

1. Clone https://gitlab.com/gitlab-da/use-cases/ai/gitlab-duo-agent-platform/challenges/challenge-cpp-observability-sensor
   1. Ask @dnsmichi for access to reproduce (access + Duo seat)
2. Open Duo Chat
3. Ask a question Please explain what this project is about
4. See a red error instead of an answer, saying I'm sorry, you have not selected a default GitLab Duo namespace. Please select a default GitLab Duo namespace in your user preferences. Error code: G3002
5. Try the same with Duo Agentic Chat Beta - works fine.

Tried mitigation

1. Set a default namespace in settings cmd , and search for gitlab)
2. Use /reset in classic Duo Chat to reset the session
3. No difference, same error.

What is the current bug behavior?

Duo Chat classic and stable should just work without a default namespace. This is a regression from previous release behavior, and breaks existing client setups requiring new configuration.

What is the expected correct behavior?

Duo Chat Classic uses the same default namespace that is assumed by Duo Agentic Chat.

Relevant logs and/or screenshots

Left pane: Classic Duo Chat, right pane: Duo Agentic Chat

Tried with setting the default namespace.

After cmd shift p and reload window same effect.

Possible fixes

Inspect the namespace change MRs in the LSP project in 8.6.0/1. gitlab-org/editor-extensions/gitlab-lsp!2042 (merged) looks like it only supports Agentic Chat

cc @viktomas @shinya.maeda @ohoral