[Fusion] Allow @require intermediates from the requiring schema

[glen-84]

Summary

• Fix RequirementsValidator to distinguish leaf fields from intermediate (navigation) fields when computing candidate source schemas for a requirement. Leaves still exclude the target schema (the gateway must source their values from elsewhere); intermediates now include all schemas, since the gateway can resolve a navigation step locally in the requiring schema as part of executing the requiring field.
• This unblocks RequiresCircular (federation-gateway-audit requires-circular), which has Post.byNovice in schema B requiring author { yearsOfExperience } where Post.author is only declared in B. The previous logic excluded B wholesale and reported Post.author as unreachable; the new logic allows the intermediate author to come from B (where it's resolved locally) while still requiring yearsOfExperience (the leaf) from outside B.

Test plan

• dotnet test src/HotChocolate/Fusion/test/Fusion.Composition.Tests --filter "FullyQualifiedName~SatisfiabilityValidatorTests|FullyQualifiedName~RequirementsValidatorTests" — 64 passed, 0 skipped, 0 failed on net8.0, net9.0, and net10.0.
• Full HotChocolate.Fusion.Composition.Tests — 498 passed, 0 skipped, 0 failed across all three TFMs.

[Copilot]

Pull request overview

This PR updates Fusion composition satisfiability validation so @require paths can use intermediate navigation fields from the requiring schema while still requiring leaf values to come from another schema. It also enables the previously skipped circular-requirements audit test.

Changes:

• Distinguishes leaf vs. intermediate fields when computing candidate schemas for requirement validation.
• Re-enables RequiresCircular as a passing satisfiability validator test.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/HotChocolate/Fusion/src/Fusion.Composition/Satisfiability/RequirementsValidator.cs	Allows intermediate requirement fields to include the excluded schema as a candidate source.
src/HotChocolate/Fusion/test/Fusion.Composition.Tests/SatisfiabilityValidatorTests.cs	Unskips the circular @require satisfiability test.

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[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.