[Task]: Ensure addons-server never assumes version channel is a binary choice

[diox]

Description

Part of Enterprise Add-ons project.

Follow-up for https://github.com/mozilla/addons/issues/14849

As a final task before enabling enterprise add-ons, we should do a pass on the entire addons-server codebase to ensure it never assumes version channel is a binary choice, since there is now a third option. In cases where we want to do something for listed versions and something else for both unlisted and enterprise, we should make sure comments are updated to note that this is an explicit choice.

Acceptance Criteria

  ### Acceptance Criteria
  - [ ] Enterprise versions are handled correctly throughout AMO 

Checks

• [X] If the issue is ready to work on, I have removed the "needs:info" label.

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