PROD-2341: sync single-item linked-content selections correctly

[jules-exel]

Summary

A linked-content dropdown that selects a single item — e.g. Game Banner's "Draw Game" field — arrived empty on the target after a sync, and the component could not be edited. Other components referencing the same list synced fine because they use whole-list links.

Investigated on a live sync 63b1dc5d-us2 (pegasus-qa clone) → 0c45c4b0-us2 (blank). Root cause was three coordinated defects in the content-push path:

1. Push ordering ignored single-item references

The content classifier (getContentItemTypes) only treated whole-list references (fulllist:true) as push-order dependencies. A single-item selection { contentid: N, fulllist: false } was invisible, so the referenced item could be pushed after the item referencing it → reference unresolvable.
Fix: new collectContentIDReferences + classifier promotes single-item contentid/sortids targets into the push-first "linked" bucket.

2. Content references were never remapped

Content-item mapping records expose targetContentID, but the field mapper read .contentID (always undefined) — so contentid/sortids remapping silently no-op'd and the payload shipped with the source id (a dangling reference the target stores as empty).
Fix: resolveTargetContentID() = targetContentID ?? contentID, applied to both the contentid and sortids branches.

3. Object form isn't accepted by the batch engine

The server batch engine reads a linked-content field with string value = row[col] as string (Agility.Shared/Engines/BatchProcessing/BatchProcessing_InsertContentItem.cs). The CLI sent the { contentid, fulllist } object, whose string cast is null → the selection stored empty (rendered as the SharedContent list name, "drawgamesassets").
Fix: emit the scalar remapped contentID string for single-item selections (isSingleItemContentSelection), leaving whole-list links (fulllist:true) and bare nested { contentid } references unchanged.

Verification

Live re-sync 63b1dc5d-us2 → 0c45c4b0-us2 (Game Banner + deps): 28/28 items with a selection populate the correct target id; 0 empty; 2 genuinely had no source selection. Confirmed by reading the field values back from the target via the management API.

Tests

• content-field-mapper.test.ts — scalar output for single-item selection, targetContentID resolution, sortids remap, and empty / whole-list / bare-reference cases left unchanged.
• get-content-item-types.test.ts — single-item + sortids targets promoted to linked; empty / unmapped / unknown-id are no-ops; existing whole-list behavior preserved.
• new collect-content-id-references.test.ts.

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