Issue #3627950: The Set domain source action stores a list of domains instead of one, so running it from a bulk form fails

Closes #3627950

The Set domain source action's configuration form rendered domain_id as checkboxes, which always submit an array, while the configuration schema, execute() and calculateDependencies() all read one domain id. It is now a radios element, so the form stores the single id they expect.

DomainSourceActionFormTest seeded an array as the saved value, which pinned the defect. It now asserts that a saved id is preselected and that submitting the form through the form builder stores exactly one id and depends on that one domain record. The submission case fails on 4.x without the change.

The test-only changes job fails on the submission case, as it should without the fix.

The automatically created domain_source_set_action.* actions never go through the form and are unaffected.

AI-Generated: Yes (Claude Code was used to write this change and its test. I reviewed them before posting.)