Fix `Users#solo_owned_organizations` returning incorrect results when user owns multiple organizations

What does this MR do and why?

Fixes a bug with Users#solo_owned_organizations returning an empty result when the user is the sole owner of multiple organizations.

Explanation

WITH "ownerships" AS (SELECT "organization_users".* FROM "organization_users" WHERE "organization_users"."user_id" = 1 AND "organization_users"."access_level" = 50)
SELECT "organizations".*
FROM "organizations"
         INNER JOIN ownerships ON ownerships.organization_id = organizations.id
WHERE (EXISTS (SELECT 1
               FROM "organization_users"
               INNER JOIN "ownerships" ON ownerships.organization_id = organization_users.organization_id
               WHERE "organization_users"."access_level" = 50
               HAVING (count(organization_users.user_id) = 1)))

This is the query generated by the current method. The bug comes from the organization_users subquery which tries to "count the owners of the organizations owned by the user, and filter them to 1". Though the problem comes from:

• The HAVING clause count(organization_users.user_id) = 1 was being applied globally across all organizations, rather than per organization. It counts how many organizations is owned by the user, instead of which organization is solo-owned by the user.
  • Solution: Add a group by clause
• There's no WHERE clause linking organizations.id to organization_users.organization_id in the subquery. This missing correlation means the WHERE EXISTS filter isn't being applied correctly.
  • Solution: Add a organization_user.organization_id = organization.id filter in the subquery. Remove the innerjoin with ownerships.

Updated Query (w/ Plan)

https://console.postgres.ai/gitlab/gitlab-production-main/sessions/32887/commands/101357

https://console.postgres.ai/gitlab/gitlab-production-main/sessions/32928/commands/101545

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Screenshots or screen recordings

How to set up and validate locally

1. Create multiple organizations and assign the user as the sole owner

user = User.first
Organizations::Organization.create!(name: 'foo', path: 'foo').add_owner(user)
Organizations::Organization.create!(name: 'bar', path: 'bar').add_owner(user)

2. Call #solo_owned_organizations to get the created organizations.

user.solo_owned_organizations.pluck(:name)
# Should return ["foo", "bar", ...]

Related to #500240 (closed)