chore: improve planning for required fields

[Noroth]

This PR adds tests and reformats the logic for planning required fields in connect

@coderabbitai summary

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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📝 Walkthrough

Walkthrough

Adds end-to-end gRPC @requires tests and refactors federation planning to create required-field RPC calls from per-operation instances, using cached entity key messages and planned selections.

Changes

gRPC @requires execution

Layer / File(s)	Summary
Federation required-field planning v2/pkg/engine/datasource/grpc_datasource/execution_plan_visitor_federation.go	Stores required-field selection sets in federation configuration, records operation-specific planned instances, and caches entity key messages.
Required-field RPC construction v2/pkg/engine/datasource/grpc_datasource/execution_plan.go	Builds required-field RPC calls from planned instances, including entity response paths, key messages, selections, and field arguments.
End-to-end execution coverage execution/engine/execution_engine_grpc_requires_test.go, v2/pkg/grpctest/schema.go	Adds scalar, list, nested, and entity-count mismatch scenarios, with HTTP/gRPC subgraph setup and expanded planner metadata.

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

Sequence Diagram(s)

sequenceDiagram
  participant GraphQLRequest
  participant FederationPlanner
  participant GRPCExecutionPlan
  participant ProductsSubgraph
  GraphQLRequest->>FederationPlanner: plan `@requires` selection
  FederationPlanner->>GRPCExecutionPlan: provide planned required-field instance
  GRPCExecutionPlan->>ProductsSubgraph: send entity required-field RPC
  ProductsSubgraph-->>GRPCExecutionPlan: return required-field result
  GRPCExecutionPlan-->>GraphQLRequest: merge result into response path

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Possibly related PRs

• wundergraph/graphql-go-tools#1452: Related gRPC @requires planning changes, including required-field arguments.
• wundergraph/graphql-go-tools#1604: Related gRPC federation planning changes and required-field alias handling.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
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.
Title check	✅ Passed	The title clearly matches the main change: improving required-field planning.
Description check	✅ Passed	The description is directly related to the code changes and test additions in this PR.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch ludwig/router-582-engine-improve-planning-for-required-fields

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