PM-5121: Stop reviewer count stepper loop

[jmgasper]

What was broken

The prior PM-5121 fix capped large reviewer counts, but QA still reproduced the freeze by closing the public review opportunity and clicking the Reviewer Count down stepper.

Root cause (if identifiable)

Closed-opportunity member selectors can leave blank additionalMemberIds slots registered in react-hook-form. When the reviewer count dropped, the trimming effect treated those blank hidden slots as data to write back during validation, allowing the reviewer watcher cleanup path to re-enter repeatedly.

What was changed

Unregistered additional member fields that are no longer visible when reviewer count shrinks, and only write/delete additional members when an actual assigned member value is being removed or preserved.

Any added/updated tests

Added HumanReviewTab coverage for decreasing reviewer count with a blank extra assignment slot and asserting no reviewer resource deletion is attempted.

Validation:

• yarn lint passed.
• yarn test:no-watch --runTestsByPath src/apps/work/src/pages/challenges/ChallengeEditorPage/components/ReviewersField/HumanReviewTab.spec.tsx passed with existing React act warnings.
• yarn run build passed with existing warnings.
• yarn test:no-watch was run and still fails in unrelated existing suites, including wallet-admin alias resolution, engagement/payment assignment utility mocks, and existing engagement schema/assignment-card expectations.