fix(cost): actual cost inflation when sizedFields parent is a non-list wrapper field

[budziam]

Problem

When a non-list wrapper field (e.g. items_page: ItemsPage!) is annotated
with @listSize(slicingArguments: ["limit"], sizedFields: ["items"]), its occurrences are never recorded
in actualListSizes because the resolver only tracks array fields there.

This caused the averaging denominator for the child list multiplier to default
to 1 (total items across all parents) instead of the number of parent field
occurrences, inflating the combined actual cost above what you'd get by summing
the same entities queried individually.

Example:

type Query {
  boards(limit: Int): [Board!]! @listSize(slicingArguments: ["limit"])
}
type Board {
  items_page(limit: Int!): ItemsPage! @listSize(slicingArguments: ["limit"], sizedFields: ["items"])
}
type ItemsPage {
  items: [Item!]!
}

{
  boards(limit: 4) {
    items_page(limit: 1) {
      items {
        id
      }
    }
  }
}

With 4 boards and 2 total items, the multiplier was computed as 2/1 = 2
instead of the correct 2/4 = 0.5.

Fix

Instead of looking up the immediate parent by chopping the JSON path string,
climb the CostTreeNode parent chain to find the nearest ancestor where
returnsListType == true, then use its actualListSizes count as the
denominator. This uses the type information already on each node rather than
inferring list-ness from map absence.

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

[coderabbitai]

📝 Walkthrough

Walkthrough

This PR refactors how parent list sizes are tracked during nested cost calculations. Instead of inferring parent list size by truncating jsonPath strings, the code now walks up the ancestor node chain to find the nearest list-returning parent and reads its actual list size. A new test validates the behavior when non-list wrapper fields separate list-returning fields.

Changes

Parent List Tracking Logic

Layer / File(s)	Summary
Cost Calculation Logic v2/pkg/engine/plan/cost.go	CostTreeNode.cost now determines parentCount by traversing node.parent to the nearest ancestor that returnsListType and reading actualListSizes[p.jsonPath], replacing the prior jsonPath string truncation method.
Test for Non-List Wrapper Parent v2/pkg/engine/plan/cost_sized_fields_test.go	New test TestActualCost_SizedFieldsParentNotTracked validates cost calculation through a chain where a non-list wrapper field (items_page) sits between two list-returning fields (boards and items), ensuring combined costs do not exceed sum of per-entity costs.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title directly and specifically describes the main change: fixing a cost calculation bug when a non-list wrapper field is annotated with @listSize.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description check	✅ Passed	The description clearly explains the problem (non-list wrapper fields not being tracked in actualListSizes), provides a concrete GraphQL example, and details the fix (climbing the parent chain instead of path truncation).

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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