refactor: improve costs code layering and readability

[ysmolski]

After many changes I have accumulated some crufty code in costs. This PR tries to add a couple of abstractions and split big methods into smaller and more logical.

[claude]

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📝 Walkthrough

Walkthrough

Refactor bundles cost computation inputs into an internal costInput and thread it through CostTreeNode.cost, costsAndMultiplier, childrenCost, and debug printing; adjust abstract-type aggregation and list-size multiplier behavior to branch between estimation and actual modes using costInput data.

Changes

Cost calculation refactor (costInput)

Layer / File(s)	Summary
costInput struct and wiring v2/pkg/engine/plan/cost.go	Adds costInput, newCostInput, returnedTypeNames; refactors CostTreeNode.cost/childrenCost to consume *costInput.
Abstract-type children cost aggregation v2/pkg/engine/plan/cost.go	Estimation takes max per-implementing-type fragment cost; actual filters implementing fragments by runtime __typename from costInput.returnedTypeNames before summing.
costsAndMultiplier and default multiplier v2/pkg/engine/plan/cost.go	Introduces costNodeResult.setDefaultMultiplier; refactors costsAndMultiplier to accept *costInput, initialize multiplier state, iterate data sources from costInput.configs, and read input-object argument values from costInput.vars.
Estimation-mode list-size & sized-field handling v2/pkg/engine/plan/cost.go	Estimation list multipliers and sized-field propagation use costInput.vars and costInput.defaultListSize; non-list nodes early-return and unset multipliers default to defaultListSize; implementing-type maxima used for sized fields.
Actual-mode list multiplier & narrowing v2/pkg/engine/plan/cost.go	Nearest enclosing list ancestor sizes read from costInput.typeStats; node multiplier computed as nodeStats.Size / enclosingSize with zero-guards; config lookups and non-list narrowing use costInput.configs and runtime distributions.
Public API entry points v2/pkg/engine/plan/cost.go	CostCalculator.EstimateCost / ActualCost build costInput via newCostInput(...) and compute rounded cost via c.tree.cost(input).
Debug printing v2/pkg/engine/plan/cost.go	DebugPrint builds costInput based on presence of typeStats; debugPrint computes subtree totals with node.cost(input), derives node intermediate values with node.costsAndMultiplier(input) + setDefaultMultiplier, reads vars via input.vars, and recurses passing input.

Estimated code review effort

🎯 4 (Complex) | ⏱️ ~60 minutes

Possibly related PRs

• wundergraph/graphql-go-tools#1461: Touches the same v2 cost computation code paths and related input-object argument cost handling.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title 'refactor: improve costs code layering and readability' directly and clearly describes the main objective of the pull request, which is a structural refactoring of the costs code to improve its organization and maintainability.
Description check	✅ Passed	The description explains the motivation for the refactoring (accumulated crufty code) and the approach taken (adding abstractions and splitting large methods), which is clearly related to the changeset of refactoring the costs code.
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.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch yury/refactor_costs

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@v2/pkg/engine/plan/cost.go`:
- Line 995: The debug output currently only prints the first root child via
c.tree.children[0].debugPrint(...), missing other top-level fields; update the
code in cost.go to iterate over c.tree.children (e.g., for _, child := range
c.tree.children { child.debugPrint(&sb, input, 0) }) so every root child is
printed, preserving the same arguments and ordering; ensure you remove the
hardcoded index usage and handle the empty children case if needed.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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📥 Commits

Reviewing files that changed from the base of the PR and between de21280 and c043b41.

📒 Files selected for processing (1)

• v2/pkg/engine/plan/cost.go