Fix admin user edit link for custom user classes

[minhluuquang]

Fixes #4010.

This updates the legacy admin users index edit action to link directly to edit_admin_user_path instead of the admin user show path. That keeps the edit URL independent of the configured Spree.user_class name and avoids relying on the show action redirect.

The existing custom user class render spec now also asserts that the edit icon points to the admin user edit route.

Tests:

• From backend/: bundle exec rspec spec/controllers/spree/admin/users_controller_spec.rb
• bundle exec standardrb backend/spec/controllers/spree/admin/users_controller_spec.rb
• bundle exec rake lint:erb

[jarednorman]

What's the purpose of this instance variable?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.68%. Comparing base (8d781ac) to head (d2e877a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6508   +/-   ##
=======================================
  Coverage   89.68%   89.68%           
=======================================
  Files         993      993           
  Lines       20863    20863           
=======================================
  Hits        18712    18712           
  Misses       2151     2151           

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