[Multi-Tenancy] Tenant list exposed without feature flag + default tenant can be deleted #6227

Description:
On main, navigating to the Security settings page exposes two critical issues related to the multi-tenancy feature:

1. Missing feature flag check: The list of tenants is visible in the Security settings even though the multi-tenancy feature has not been released yet. A feature flag check is missing on this page.
2. Default tenant can be deleted: It is possible to delete the default tenant (the only tenant in the list), which should not be allowed. The current implementation should only permit renaming the default tenant, not deleting it.
3. Default tenant can have new tenants added to it: It is also possible to add new tenants from this view, which should not be available at this stage.

Reproducible Steps:

1. Navigate to https://main.oaev.staging.filigran.io/<instance-id>/admin/settings/security/
2. Observe the tenant list is visible despite the feature not being released
3. Attempt to delete the default tenant
4. Observe the deletion is not blocked

Expected Output:

• The tenant list should not be visible without the appropriate feature flag enabled
• Deleting the default tenant should be blocked; only renaming should be allowed
• Adding new tenants should not be available from this view

Actual Output:

• The tenant list is visible without the feature flag
• The default tenant can be deleted
• New tenants can be added

Environment: main (main.oaev.staging.filigran.io)