Fix Duo Enterprise add-on check to use seat assignment instead of namespace membership

What does this MR do and why?

Fixes the Duo Enterprise add-on check in Ai::DuoCodeReview::Modes::Dap to correctly validate seat assignments instead of namespace membership.

The bug was in user_has_duo_enterprise_add_on? which used for_active_add_ons([:duo_enterprise], user). This method checks if the user is a member of a namespace with active Duo Enterprise purchases, not whether the user has been assigned a seat.

This is incorrect because:

• A user can be a namespace member without having a seat assigned
• The check should validate actual seat assignment, not just membership

More context: https://gitlab.com/gitlab-org/gitlab/-/issues/587404#note_3048444301

How to set up and validate locally

1. Create a namespace with Duo Enterprise add-on
2. Add a user as a member and assign them a seat
3. Verify the user cannot access DAP code review (classic duo code review should be used)
4. Un-assign the user a seat
5. Verify the user can now access DAP code review

Screenshots

Seat Assigned	Seat Un-assigned

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Related to https://gitlab.com/gitlab-org/gitlab/-/issues/587404