fix: scope orderable join reordering by parent relation (#15842)

## Context
The root problem was scope leakage in join-based ordering. Reorder
operations were effectively treating join items as one shared list, so
moving an item in one parent context could influence ordering in a
different parent context.

## Changes
- Reorder key generation, adjacent-row lookup, and initial missing-key
backfill now run in the same join `on` scope as the target item.
- Fast drag/drop cases that send `pending` now resolve against scoped
context before computing new keys.
- `index.ts` keeps orchestration, and scoped query logic is extracted
into focused helpers to keep behavior changes easier to review.
- Replaced a local where-wrapper with the shared
`combineWhereConstraints` utility.

## Impact
Users can reorder join rows without cross-parent side effects. Items in
unrelated parent relations no longer jump or get new keys from another
parent's ordering context.

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Co-authored-by: German Jablonski <GermanJablo@users.noreply.github.com>