Boost unit test coverage for `consume.ts`

Add unit test coverage for `consume.ts` that at minimum tests simple functions and sets up required state (think of this ticket as a kind of prototype for what would be required to cover more deeply, and a small % win).

Some options for small refactoring wins: Move command registration out of this file? Add a Message Viewer Provider class for state? See also tickets under https://github.com/confluentinc/vscode/issues/2049 which we may want to link this under.

Since it might take some large refactoring to boost coverage to our goal of 80% per file, E2E tests are a path forward to give confidence for feature additions and maintenance. [Boosting E2E coverage](https://github.com/confluentinc/vscode/issues/3252) can be done in parallel