PM-5481: Fix MM submissions leaderboard visibility

[jmgasper]

What was broken

• The MM submissions tab could collapse back to the capped submissions embedded in challenge details, so authenticated unregistered users saw only submitters from the recent submissions page.
• Final MM ranks and scores could render during an open submission phase when raw submission payloads included finalScore values.

Root cause (if identifiable)

• mapStateToProps rebuilt mmSubmissions from challenge.submissions whenever review summations existed, ignoring the fully paginated MM submissions already fetched into state.
• Final result visibility treated any loaded final score or rank as displayable without checking whether submissions were still open.

What was changed

• Prefer the fully fetched mmSubmissions state before falling back to challenge details data.
• Hide MM final ranks and scores while any submission phase is open, and prevent hidden final scores from affecting sort order.

Any added/updated tests

• Added regression coverage for preserving fully fetched MM submitters when review summations exist.
• Added final result visibility coverage for open submission phases.