[VSCode] Cover Agentic Chat with E2E tests

Problem

Our E2E tests now test only for code suggestions and account setup. It's great to know on each build that these two features work but it would be great to get more confidence in the extension working as expected

Solution

Write e2e tests covering Agentic Chat functionality, starting with a happy path

1. Agentic chat is present and can be opened
2. A request can be typed in and sent.

Implementation details

See duo_chat.e2e.js for example implementation

/cc @jay_mccure