What was broken
Challenge update requests could include edited phase scheduledEndDate values, but the API dropped those fields during sanitization and recalculated phase ends from the previous duration.

Root cause (if identifiable)
The earlier PM-5138 fix updated derived submission date mapping, but the challenge update path still only allowed scheduledStartDate and duration through for phases. When Work sent an edited registration or submission end date, populatePhasesForChallengeUpdate never saw it.

What was changed
Allowed scheduledEndDate in the challenge update phase schema and sanitizer. Phase schedule recalculation now applies an explicit scheduledEndDate when provided, derives duration from the resulting start/end range, and cascades successor starts from the preserved predecessor end.

Any added/updated tests
Added phase-helper unit coverage for Development-style Registration/Submission phases and MM Registration/MM Submission phases with edited scheduled end dates.

Validation

• pnpm lint passed.
• Focused PM-5138 tests passed: pnpm test -- --grep "phase helper|prisma helper|challenge response helper".
• pnpm exec standard test/unit/phase-helper.test.js test/unit/prisma-helper.test.js test/unit/challenge-helper.test.js passed.
• pnpm build could not run because this package has no build script or build executable.
• Full pnpm test was attempted first without DATABASE_URL and failed at DB-backed setup. After starting the local PostgreSQL container and running prisma db push on a disposable local schema, the suite progressed but still failed in existing unrelated validation/mock expectation tests.