feat(checkout): ✨ Add parent/parentRelationship property to CartLines for Checkout.

[patrickDouglas]

Background

This PR adds parent/parentRelationship properties to CartLines for Checkout, enabling the representation of relationships between cart lines. Follow up to the private repo change.

Solution

Added support for parent-child relationships between cart lines in the Checkout API:

1. Added a new CartLineParentRelationship interface that contains parent line information
2. Added the parentRelationship property to the CartLine interface
3. Added optional parent property to CartLineAddChange and CartLineUpdateChange interfaces to allow setting parent relationships when adding or updating cart lines
4. Exported the new CartLineParentRelationship type

These changes enable extensions to work with hierarchical cart line structures, such as bundles or add-ons that are associated with a parent product.

🎩

• Test adding a cart line with a parent relationship
• Test updating a cart line to associate it with a parent
• Verify parent relationships are correctly displayed in the UI

Checklist

• I have 🎩'd these changes
• I have updated relevant documentation

[patrickDouglas]

• feat(checkout): ✨ Add parent/parentRelationship property to CartLines for Checkout. #3072 👈 (View in Graphite)
• 2025-10-rc

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[lrsterrett]

Everything looks right to me. I can't remember but did you end up tophatting the locally generated docs to see if this property showed up correctly?

[patrickDouglas]

Everything looks right to me. I can't remember but did you end up tophatting the locally generated docs to see if this property showed up correctly?

@lrsterrett I did 😄 It looked good to me!

[patrickDouglas]

How many ✅ do y'all typically like to have per release? Is one enough?

[patrickDouglas]

Merge activity

• Jul 14, 9:16 PM UTC: A user started a stack merge that includes this pull request via Graphite.
• Jul 14, 9:17 PM UTC: Graphite rebased this pull request as part of a merge.
• Jul 14, 9:18 PM UTC: @patrickDouglas merged this pull request with Graphite.