HOTFIX-PM5755: Resolve phase-name-only reviewer defaults

[jmgasper]

Summary

• Preserve phaseName from the default-reviewer API response.
• Resolve phase-name-only reviewer defaults against the selected challenge timeline before applying the legacy fallback.
• Cover the production response shape in the reviewer reconciliation and service tests, and update the editor documentation.

Root cause

The follow-up to #2161 correctly requested defaults for the selected timeline, but the live default-reviewer records identify their phases with phaseName; their phaseId values are null. Platform UI discarded phaseName, so every default fell back to the same challenge phase. The completeness check then treated the other required phases as unavailable and left the hidden reviewer rows unchanged, which caused draft validation to report: Assign all required members when public review opportunity is closed.

Impact

Two-round Design challenge drafts can resolve all five default reviewer rows. Checkpoint Review, Review, and Approval are assigned to the selected copilot, while Screening and Checkpoint Screening remain optional.

Validation

• yarn test:no-watch --runInBand src/apps/work/src/lib/services/challenges.service.spec.ts src/apps/work/src/pages/challenges/ChallengeEditorPage/components/ReviewersField/HumanReviewTab.spec.tsx (39 tests passed)
• yarn lint
• yarn run build (passed with existing repository warnings)