test: Clear before asking duo chat e2e test

Description

Clears conversation with duo chat before performing duo chat test. This ensures that we get a predictable response from duo chat (if we keep asking hi of gitlab duo chat it starts to get creative with responses). Also reduces assertion to GitLab rather than GitLab Duo Chat to make it more reliable.

This aims to fix such failures as seen in: https://gitlab.com/gitlab-org/gitlab-vscode-extension/-/jobs/7152618084 e.g.

Screenshots (if appropriate)

What CHANGELOG entry will this MR create?

• fix: Bug fix fixes - a user-facing issue in production - included in changelog
• feature: New feature - a user-facing change which adds functionality - included in changelog
• BREAKING CHANGE: (fix or feature that would cause existing functionality to change) - should bump major version, mentioned in the changelog
• None - other non-user-facing changes