Fix Fusion composition crash on repeated @key in Apollo Federation schemas

[michaelstaib]

Fixes #10221

[Copilot]

Pull request overview

This PR addresses Fusion composition crashing when an Apollo Federation schema repeats the same @key(fields: ...) on both a type and an extend type block (issue #10221). It does so by normalizing key field selections to detect duplicates and by deduplicating generated lookup fields instead of throwing an unhandled ArgumentException.

Changes:

• Deduplicate repeated @key directives (including those differing only by whitespace/field order) when generating Fusion lookup fields, preventing duplicate fusion__lookup_* collisions.
• Introduce FieldSelection normalization logic and reuse it in composition validation to compare structurally equivalent key selections.
• Switch Utf8GraphQLParser.Syntax fragment parsing to use ParserOptions.Trusted (no request limits) and add/extend tests + snapshots covering the new behaviors.

Reviewed changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/HotChocolate/Language/test/Language.Tests/Parser/GraphQLParserSyntaxTests.cs	Adds a test ensuring Utf8GraphQLParser.Syntax.ParseSelectionSet is not constrained by default request limits.
src/HotChocolate/Language/src/Language.Utf8/Utf8GraphQLParser.Syntax.cs	Uses ParserOptions.Trusted for Syntax fragment parsing and documents the intent (trusted input).
src/HotChocolate/Fusion/test/Fusion.Composition.Tests/PreMergeValidationRules/InterfaceObjectKeyMismatchRuleTests.cs	Adds coverage for key equivalence when order/whitespace differ.
src/HotChocolate/Fusion/test/Fusion.Composition.Tests/FieldSelectionTests.cs	New test suite for field selection normalization behavior.
src/HotChocolate/Fusion/test/Fusion.Composition.ApolloFederation.Tests/RepeatedKeyLookupTests.cs	Repro-style test ensuring composition succeeds for repeated type+extension @key.
src/HotChocolate/Fusion/test/Fusion.Composition.ApolloFederation.Tests/FederationSchemaTransformerTests.cs	Adds transformer tests ensuring single lookup generation for repeated/normalized keys, and correctness around resolvable.
src/HotChocolate/Fusion/test/Fusion.Composition.ApolloFederation.Tests/FederationKeyValidationTests.cs	Adds coverage that invalid @key(fields: ...) syntax produces a diagnostic instead of crashing.
src/HotChocolate/Fusion/test/Fusion.Composition.ApolloFederation.Tests/snapshots/RepeatedKeyLookupTests.Compose_Should_Succeed_When_SameKeyIsRepeatedOnTypeAndExtension.graphql	Snapshot for composed schema output in repeated-key scenario.
src/HotChocolate/Fusion/test/Fusion.Composition.ApolloFederation.Tests/snapshots/FederationSchemaTransformerTests.Transform_Should_KeepBothKeys_When_SameFieldsDifferOnlyInResolvable.md	Snapshot validating key retention rules when resolvable differs.
src/HotChocolate/Fusion/test/Fusion.Composition.ApolloFederation.Tests/snapshots/FederationSchemaTransformerTests.Transform_Should_GenerateSingleLookup_When_SameKeyIsRepeatedOnTypeAndExtension.md	Snapshot validating single lookup generation for repeated type+extension key.
src/HotChocolate/Fusion/test/Fusion.Composition.ApolloFederation.Tests/snapshots/FederationSchemaTransformerTests.Transform_Should_GenerateSingleLookup_When_ListSegmentKeyIsRepeated.md	Snapshot validating dedupe behavior for repeated list-segment keys.
src/HotChocolate/Fusion/test/Fusion.Composition.ApolloFederation.Tests/snapshots/FederationSchemaTransformerTests.Transform_Should_GenerateSingleLookup_When_KeysDifferOnlyInWhitespaceOrFieldOrder.md	Snapshot validating dedupe behavior for equivalent keys with differing formatting/order.
src/HotChocolate/Fusion/src/Fusion.Composition/PreMergeValidationRules/InterfaceObjectKeyMismatchRule.cs	Switches key comparison to FieldSelection.Normalize.
src/HotChocolate/Fusion/src/Fusion.Composition/FieldSelection.cs	Introduces normalization/merging/sorting logic for selection sets used as @key(fields: ...).
src/HotChocolate/Fusion/src/Fusion.Composition/ApolloFederation/GenerateLookupFields.cs	Adds duplicate-key identity tracking and reuses parsed selection sets to prevent duplicate lookup generation.

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Patch coverage

81.1% of changed lines covered (189/233)

File	Covered	Changed	Patch %
…/Language/src/Language.Utf8/Utf8GraphQLParser.Syntax.cs	7	12	58.3% 🔴
…/HotChocolate/Fusion/src/Fusion.Composition/FieldSelection.cs	165	204	80.9% 🟡
…/Fusion.Composition/ApolloFederation/GenerateLookupFields.cs	16	16	100.0% 🟢
…/PreMergeValidationRules/InterfaceObjectKeyMismatchRule.cs	1	1	100.0% 🟢

Uncovered changed lines (JSON)

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Project coverage: 54.5% (243533/446855 lines)