Fix namespaces where namespaces.path is not equal to users.namespace

We have reports from customers that for some users the username doesn't match its user namespace.path which results in issues. This also occurs on gitlab.com https://console.postgres.ai/gitlab/gitlab-production-main/sessions/29089/commands/90598.

We need to ensure that we have no user namespace path that doesn't match its username.

Implementation plan

1. Add a validation to ensure the user namespace path equals the username.
2. Add a migration to fix existing records.