fix: improve handling of nullable lists for required fields

[Noroth]

This PR adds tests for nullable lists as arguments for @requires fields. It also improves the behavior when nullable vs empty lists are provided as arguments

Checklist

• I have discussed my proposed changes in an issue and have received approval to proceed.
• I have followed the coding standards of the project.
• Tests or benchmarks have been added or updated.

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Summary by CodeRabbit

• Bug Fixes

  • Corrected handling of nullable lists so empty lists remain available while explicitly null lists remain unset.
  • Improved federation field resolution for nullable list values, including object, interface, and union results.

• New Features

  • Added support for optional storage histories, recommended items, and tag filtering by list lengths.
  • Added GraphQL and service support for nullable-list arguments and responses.

• Tests

  • Added comprehensive coverage for populated, empty, and null list scenarios.

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 634e76b and d63d2bd.

📒 Files selected for processing (1)

• v2/pkg/engine/datasource/grpc_datasource/compiler.go

🚧 Files skipped from review as they are similar to previous changes (1)

• v2/pkg/engine/datasource/grpc_datasource/compiler.go

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📝 Walkthrough

Walkthrough

The change adds nullable-list @requires fields and arguments across GraphQL schemas, protobuf contracts, Connect handlers, mock services, mappings, compiler traversal, execution-plan tests, and federation integration tests.

Changes

Nullable @requires lists

Layer / File(s)	Summary
Nullable-list contracts and Connect surface v2/pkg/grpctest/product.proto, v2/pkg/grpctest/productv1/productv1connect/product.connect.go, v2/pkg/grpctest/schema.go, v2/pkg/grpctest/testdata/products.graphqls	Added list wrapper messages, four storage requirement RPCs, Connect client and handler support, GraphQL field definitions, and federation requirement metadata.
Mock RPC implementations and mappings v2/pkg/grpctest/mockservice_requires.go, v2/pkg/grpctest/mockservice_connect.go, v2/pkg/grpctest/mapping/mapping.go, v2/pkg/engine/datasource/grpc_datasource/mapping_test_helper.go	Added nullable wrapped-list responses, nullable list-length filtering, Connect passthroughs, and storage field mappings.
List traversal and execution planning v2/pkg/engine/datasource/grpc_datasource/compiler.go, v2/pkg/engine/datasource/grpc_datasource/execution_plan_requires_test.go	Preserved non-null empty list wrappers and added plan coverage for object, interface, union, and nullable argument cases.
Federation execution validation v2/pkg/engine/datasource/grpc_datasource/grpc_datasource_federation_test.go	Added execution coverage for populated, empty, and null lists, abstract types, and nullable list arguments.

Estimated code review effort: 4 (Complex) | ~45 minutes

Sequence Diagram(s)

sequenceDiagram
  participant GraphQLClient
  participant GRPCDatasource
  participant MockServiceConnect
  participant MockService
  GraphQLClient->>GRPCDatasource: Execute nullable-list `@requires` field
  GRPCDatasource->>MockServiceConnect: Call storage requirement RPC
  MockServiceConnect->>MockService: Forward request and metadata
  MockService-->>MockServiceConnect: Return populated, empty, or null wrapper
  MockServiceConnect-->>GRPCDatasource: Return Connect response
  GRPCDatasource-->>GraphQLClient: Resolve nullable-list result

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Possibly related PRs

• wundergraph/graphql-go-tools#1452: Extends @requires argument handling and nullable-list behavior.
• wundergraph/graphql-go-tools#1630: Modifies traverseList and adds overlapping nullable-list @requires tests.
• wundergraph/graphql-go-tools#1613: Adds related nullable and abstract list handling for @requires.

<f­­ixed_issue_severity>Low</fixed_issue_severity>

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main change: improved handling of nullable lists for required fields.
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 ludwig/router-606-engine-add-test-cases-for-list-wrappers-for-required-fields

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[fiam]

LGTM