Querying Memberships Fix: ensure distinct memberships when querying for ones own public memberships

[yuenmichelle1]

Bug found while working on User Groups Features

Correspondence on issue found in this Slack thread here: https://zooniverse.slack.com/archives/C010QAPB67J/p1715888434468079?thread_ts=1715813389.436219&cid=C010QAPB67J

TL;DR; of the issue is that when doing a GET request to /memberships (either via /memberships?user_id=_user_group_id=_ or via /memberships/id) to one's own public membership, response responds with duplicates.

• The reason is that when querying memberships (in membership policy) we add a union of any public memberships. This leads to duplicates when one is querying one's own public membership.
• Because of this duplicates issue, we cannot deactivate memberships through DELETE because ETag headers will never match the precondition (due to response of GET request having dupes). Which results in 412 Precondition Failed error.

Changes:

• Ensure distinct memberships returned in query
• Updating membership policy specs to ensure duplicates are not return using a contains_exactly rspec matching array method vs includes match

Review checklist

• First, the most important one: is this PR small enough that you can actually review it? Feel free to just reject a branch if the changes are hard to review due to the length of the diff.
• If there are any migrations, will they the previous version of the app work correctly after they've been run (e.g. the don't remove columns still known about by ActiveRecord).
• If anything changed with regards to the public API, are those changes also documented in the apiary.apib file?
• Are all the changes covered by tests? Think about any possible edge cases that might be left untested.

[zwolf]

LGTM, just a comment on the readability of those long argument lists.