Enhancement: A/B test variants now match the original's type

When creating an A/B test, the variant picker now only offers components of the same type as the component you're testing.

Why. An upsell tested against a content banner can't render in the same placement, which made results misleading and could leave customers seeing nothing at all on one side of the test. Matching types keeps every test a fair, like-for-like comparison.

Existing setups. Running tests are unaffected; the constraint applies when you create or edit a test.