fix: improve handling of nullable lists for required fields

[Noroth]

This PR adds test cases for handling nullable lists as arguments for @requires fields. It also improves the behavior for passing nullable lists as arguments to distinguish between empty and null lists.

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

• New Features

  • Added support for additional storage-related operations, including recommended items, operation history, metadata history, latest operations, and tag filtering by lengths.
  • Expanded federation support for fields returning interfaces, unions, nested abstract values, and nullable lists.
  • Added handling for nullable list arguments, distinguishing absent, null, empty, and populated values.

• Bug Fixes

  • Corrected optional list processing so empty lists are preserved instead of being treated as omitted.

[coderabbitai]

Caution

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📥 Commits

Reviewing files that changed from the base of the PR and between b90db54 and 2518c8b.

⛔ Files ignored due to path filters (2)

• v2/pkg/grpctest/productv1/product.pb.go is excluded by !**/*.pb.go
• v2/pkg/grpctest/productv1/product_grpc.pb.go is excluded by !**/*.pb.go

📒 Files selected for processing (14)

• v2/pkg/engine/datasource/grpc_datasource/compiler.go
• v2/pkg/engine/datasource/grpc_datasource/execution_plan.go
• v2/pkg/engine/datasource/grpc_datasource/execution_plan_requires_test.go
• v2/pkg/engine/datasource/grpc_datasource/execution_plan_visitor.go
• v2/pkg/engine/datasource/grpc_datasource/execution_plan_visitor_federation.go
• v2/pkg/engine/datasource/grpc_datasource/grpc_datasource_federation_test.go
• v2/pkg/engine/datasource/grpc_datasource/mapping_test_helper.go
• v2/pkg/grpctest/mapping/mapping.go
• v2/pkg/grpctest/mockservice_connect.go
• v2/pkg/grpctest/mockservice_requires.go
• v2/pkg/grpctest/product.proto
• v2/pkg/grpctest/productv1/productv1connect/product.connect.go
• v2/pkg/grpctest/schema.go
• v2/pkg/grpctest/testdata/products.graphqls

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📝 Walkthrough

Walkthrough

The PR adds gRPC federation @requires support for abstract return types and nullable lists. It refactors composite selection planning, adds Storage RPC contracts and mock handlers, preserves null versus empty list behavior, and expands execution-plan and datasource tests.

Changes

Federation @requires support

Layer / File(s)	Summary
Schema and RPC contracts v2/pkg/grpctest/product.proto, v2/pkg/grpctest/schema.go, v2/pkg/grpctest/testdata/products.graphqls, v2/pkg/grpctest/productv1/productv1connect/product.connect.go	Adds Storage fields, list wrappers, required-field RPC messages, Connect clients, handlers, routes, and defaults.
Composite execution planning v2/pkg/engine/datasource/grpc_datasource/compiler.go, v2/pkg/engine/datasource/grpc_datasource/execution_plan.go, v2/pkg/engine/datasource/grpc_datasource/execution_plan_visitor*.go, v2/pkg/engine/datasource/grpc_datasource/execution_plan_requires_test.go	Builds reusable composite type configuration and preserves abstract member, fragment, and nullable-list metadata in required-field plans.
Required-field RPC execution v2/pkg/grpctest/mapping/mapping.go, v2/pkg/engine/datasource/grpc_datasource/mapping_test_helper.go, v2/pkg/grpctest/mockservice_*.go, v2/pkg/grpctest/productv1/productv1connect/product.connect.go	Adds Storage mappings, Connect passthroughs, and mock implementations for recommended items, operations, metadata history, and tag-length filtering.
Federation datasource validation v2/pkg/engine/datasource/grpc_datasource/grpc_datasource_federation_test.go	Tests interface and union results, nested abstract values, fragments, concrete __typename fields, nullable lists, and nullable list arguments through the gRPC datasource pipeline.

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

Possibly related PRs

• wundergraph/graphql-go-tools#1452: Related federation @requires planning and nullable list argument handling.
• wundergraph/graphql-go-tools#1606: Directly related gRPC federation @requires execution-plan construction.
• wundergraph/graphql-go-tools#1613: Related abstract-type planner refactor and @requires tests.

✨ 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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