Issue #3589210: Add config schema for the domain_admin entity reference selection handler

Summary

The DomainAdminSelection entity reference selection plugin (id: domain_admin) extends DomainSelection which extends core's DefaultSelection, but domain.schema.yml did not declare a schema entry for the subclass plugin id. As a result the Configuration Inspector reports three "missing schema" rows on every field configured with the domain_admin handler — most notably field.field.user.user.field_domain_admin shipped by this module.

Addressed with two minimal edits in the domain submodule:

1. domain/config/schema/domain.schema.yml — type alias to entity_reference_selection.default, since the subclass exposes exactly the same handler settings as the parent.
2. domain/config/install/field.field.user.user.field_domain_admin.yml — realigned to the modern settings.handler_settings.* nesting with explicit auto_create: false (boolean), so fresh installs match the schema without depending on core's legacy normalisation.

Closes the three "missing schema" entries reported by Configuration Inspector for every site (existing and fresh).

Residual case (existing sites)

Long-lived sites that historically saved auto_create: 0 as integer (legacy form-value behaviour) will still see one residual error after this MR lands:

settings.handler_settings.auto_create: variable type is integer but applied schema class is BooleanData

Documented procedure to clear it (no update hook required, see issue): visit admin/config/people/accounts/fields/user.user.field_domain_admin and click Save settings. The merge of defaultConfiguration() casts auto_create back to boolean.

Verification

Tested on Drupal 11.3.9 + domain 3.x + Configuration Inspector. After applying the patch and re-saving the field config, the inspector returns no errors for field.field.user.user.field_domain_admin.

Test plan

• Pipeline passes.
• Manual smoke-test on a fresh install: new field_domain_admin is created with auto_create: false (boolean) and handler_settings correctly nested.