Issue #3627950: The Set domain source action stores a list of domains instead of one, so running it from a bulk form fails (backport to 3.0.x)

Backport of !472 (merged) (#3627950) to 3.0.x.

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.

The source change is the same as on 4.x. DomainSourceActionFormTest is the 4.x version, keeping the @group domain_source annotation this branch still needs for run-tests.sh on the cores it supports.

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

AI-Generated: Yes (Claude Code was used to write this backport. I reviewed it before posting.)