PM-5457: Backfill duplicate challenge history display fields

[jmgasper]

What was broken
Development challenge history cards could render without placement or rating even when the same challenge had those display values under a configured rating path such as DATA_SCIENCE / AI Engineering.

Root cause (if identifiable)
The stats history response grouped duplicate challenge rows by track and type without sharing display fields between the source-track row and the rating-path row. Profile pages that displayed the source-track history saw the metadata-only row and rendered blank stats.

What was changed
The unified stats history response now backfills only missing placement and newRating values from another history row with the same challenge id. Existing values are preserved, so a source-track rating is not overwritten by a rating-path rating.

Any added/updated tests
Added a stats history unit test covering duplicate Development / Challenge and DATA_SCIENCE / AI Engineering rows, including preserving an existing Development rating while copying missing placement.

Validation run:

• pnpm exec mocha -t 20000 test/unit/StatsDimensionHelper.test.js --exit passed.
• pnpm lint passed.
• pnpm test ran with placeholder Prisma URLs and reported 152 passing tests, then failed four DB-backed member service hooks because no PostgreSQL server was available at localhost:5432.
• pnpm build could not run because this package does not define a build command.