[VSCode] Add terminal context status to diagnostics page

From MR comment: !2515 (comment 2441089795)

Problem to solve

In feat: add status checks for Duo Chat explain_te... (gitlab-org/editor-extensions/gitlab-lsp!1569 - merged) and feat: add terminal_context status check (!2515 - merged) we added a new status check for the explain terminal context feature.

Customers may have problems or questions related to the availability of this feature, so clearly seeing the state of the status check would be helpful.

Proposed solution

Add the new status check state to the diagnostics page.

Since this is a part of chat, it might make sense to include the state check in the ## GitLab Duo Chat section and NOT AS IT'S OWN ENTITY that is displayed below (in a quick POC that state checks are working as expected):