fix: accept empty lists for non-null list items

[jensneuse]

Summary

• Accept empty list defaults for GraphQL list types with non-null items in operation-level validation.
• Preserve validation of invalid non-empty list elements.
• Add regression coverage for [] defaults and invalid enum-list defaults.

Links

• Linear: https://linear.app/wundergraph/issue/ENG-9882/string-cannot-represent-value
• Cosmo issue: String cannot represent value: [] cosmo#3095

Verification

• go test ./pkg/astvalidation/reference/testsgo -run "^TestValuesOfCorrectTypeRule$" -count=1
• go test ./... from v2
• Cosmo staged regression and race test passed against this fixed local module revision.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 18f4bdd3-2c46-4bc5-92e9-2c08df7341cd

📥 Commits

Reviewing files that changed from the base of the PR and between bbb3db1 and 65cb1cf.

📒 Files selected for processing (2)

• v2/pkg/astvalidation/operation_rule_values.go
• v2/pkg/astvalidation/reference/testsgo/ValuesOfCorrectTypeRule_test.go

---

📝 Walkthrough

Walkthrough

Empty lists with non-null item types are now accepted during operation value validation. Reference tests add valid scalar and empty-list variable defaults, and verify enum errors for invalid list defaults.

Changes

List value validation

Layer / File(s)	Summary
Accept empty non-null-item lists v2/pkg/astvalidation/operation_rule_values.go	valueSatisfiesOperationListType returns true for empty lists when the item type is non-null.
Expand variable default tests v2/pkg/astvalidation/reference/testsgo/ValuesOfCorrectTypeRule_test.go	Valid defaults include Int! and empty [String!] values; invalid defaults include an unknown DogCommand enum value and its expected validation error.

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 concisely matches the main change: accepting empty lists for non-null list items.
Description check	✅ Passed	The description matches the code changes and test additions, including empty-list defaults and enum-list validation.
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.

✨ 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 jens/eng-9882-string-cannot-represent-value

---

_{Comment @coderabbitai help to get the list of available commands.}