Fix gem transfer when co-owners are already organization members

[anton-bondaronok]

Description

Closes #6740

Transfer used to always insert new memberships from approved invites. That failed when a co-owner was already in the target org: the duplicate Membership was invalid and unsaved, then OrganizationMailer.user_invited raised ActiveJob::SerializationError.

Existing members are now skipped. They stay off the users step, their org role is unchanged, and they are not emailed. Their individual gem ownership is still removed so they access the gem through the organization. Role changes continue to go through organization management. New invitees are created and emailed as before.

How to test

1. Transfer a gem whose co-owner is already in the org (seeded rubygem1 / gem-maintainer to RubyGems). Confirm succeeds; membership role is unchanged; gem ownership is removed; no invite email; the user does not appear on the users step.
2. Mix of new invitee + existing member: new membership created + emailed; existing member unchanged except gem ownership removed.
3. Non-member set to Outside Contributor: no membership; gem ownership demoted to maintainer.
4. bin/rails test test/models/rubygem_transfer_test.rb test/functional/rubygems/transfer/users_controller_test.rb

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.77%. Comparing base (baa58ad) to head (69b5c31).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6768   +/-   ##
=======================================
  Coverage   97.77%   97.77%           
=======================================
  Files         531      531           
  Lines       11443    11450    +7     
=======================================
+ Hits        11188    11195    +7     
  Misses        255      255           

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[brian-penguin]

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