PM-5446: Restore persisted active schedules

[jmgasper]

What was broken

• Active challenge saves could show a success-only state after an attempted active phase shortening, even when challenge-api kept the original timeline.
• The view page could temporarily show the shortened timeline until a refresh restored the persisted schedule.

Root cause

• The editor reset and navigated with the patch response, which could echo submitted phase dates instead of the persisted schedule state.

What was changed

• Active scheduled challenge saves now refetch the persisted challenge before resetting or navigating.
• Rejected active-phase shortening is detected from the fresh schedule and shown as a partial-save warning while other saved fields remain saved.
• Challenge editor docs were updated for the post-save schedule verification behavior.

Any added/updated tests

• Added a ChallengeEditorForm regression test for a patch response that echoes shortened phase dates while the persisted timeline remains unchanged.
• Validation run: focused PM-5446 test passed; lint passed; build passed with existing warnings. Full yarn test:no-watch --runInBand still fails on unrelated existing wallet-admin alias/icon issues and pre-existing launch approval expectations.