Support @tag on directive definitions

[glen-84]

Summary

• Adds DIRECTIVE_DEFINITION as a valid location for the @tag directive, so directive definitions can carry @tag metadata via schema-first SDL, code-first (.Tag() on IDirectiveTypeDescriptor), and implementation-first ([Tag] on the directive class). Gated out in Apollo Federation mode to stay consistent with Apollo's @tag definition.
• In Fusion composition: the canonical @tag definition includes DIRECTIVE_DEFINITION; tag-based exclusion (--exclude-by-tag) removes a tagged directive definition and cascades to every application of it across the schema; and source directive definitions are matched against the canonical by location subset, so schemas declaring @tag (or any merger-backed directive) at a subset of the canonical's locations still compose, keeping existing subgraphs backward compatible.

Test plan

• Core: schema-first, code-first, and implementation-first tests apply @tag at every supported location including directive definitions; an Apollo Federation guard asserts DIRECTIVE_DEFINITION is excluded there.
• Fusion: composition tests cover the canonical @tag location, the exclude-by-tag cascade across all application sites (schema, types, fields, arguments, enum values, input fields, directive definitions), and location-subset backward compatibility.

[Copilot]

Pull request overview

Adds support for applying @tag to directive definitions (DIRECTIVE_DEFINITION) across HotChocolate Core and Fusion composition, while keeping Apollo Federation behavior aligned with Apollo’s canonical @tag (i.e., excluding DIRECTIVE_DEFINITION in federation mode). It also extends Fusion’s exclude-by-tag preprocessing to remove tagged directive definitions and cascade the removal to all applications, and relaxes directive-definition matching during merge to allow source schemas that declare a subset of canonical locations.

Changes:

• Extend the canonical @tag directive (Core + Fusion) to include DIRECTIVE_DEFINITION, and enable tagging directive definitions via schema-first, code-first (IDirectiveTypeDescriptor.Tag(...)), and implementation-first ([Tag] on directive classes).
• In Fusion preprocessing, support exclude-by-tag removal of tagged directive definitions and remove all applications of removed directives across the schema.
• In Fusion merging, treat source directive definitions as compatible with canonical definitions when their declared locations are a subset of canonical (arguments/repeatability must still match).

Reviewed changes

Copilot reviewed 17 out of 17 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/HotChocolate/Fusion/test/Fusion.Composition.Tests/SourceSchemaPreprocessorTests.cs	Adds coverage for excluding tagged directive definitions and cascading removal to all directive application sites.
src/HotChocolate/Fusion/test/Fusion.Composition.Tests/SourceSchemaMerger.TagDirective.Tests.cs	Updates merge tests to cover DIRECTIVE_DEFINITION and location-subset compatibility, plus a custom directive tagging scenario.
src/HotChocolate/Fusion/test/Fusion.Composition.Tests/snapshots/SourceSchemaPreprocessorTests.Preprocess_ExcludeByTag_RemovesTaggedDirectiveDefinitionAndCascadesToAllApplicationSites.graphql	New snapshot validating exclude-by-tag cascade results.
src/HotChocolate/Fusion/src/Fusion.Composition/SourceSchemaPreprocessor.cs	Implements removal of tagged directive definitions and cascaded removal of their applications.
src/HotChocolate/Fusion/src/Fusion.Composition/SourceSchemaMerger.cs	Switches directive-definition merge gating to compatibility (subset-of-locations) instead of strict equality.
src/HotChocolate/Fusion/src/Fusion.Composition/DirectiveDefinitionNodeComparer.cs	Removes the old strict structural comparer (no longer used).
src/HotChocolate/Fusion/src/Fusion.Composition/DirectiveDefinitionCompatibility.cs	Adds compatibility check allowing source location subsets of canonical directive definitions.
src/HotChocolate/Fusion/src/Fusion.Composition/Definitions/TagMutableDirectiveDefinition.cs	Updates Fusion’s canonical @tag locations to include DIRECTIVE_DEFINITION.
src/HotChocolate/Core/test/Types.Tests/Types/Directives/TagDirectiveTests.cs	Expands tests to cover schema-first, code-first, and implementation-first tagging including directive definitions.
src/HotChocolate/Core/test/Types.Tests/Types/Directives/snapshots/TagDirectiveTests.SchemaFirst_Tag.graphql	Updates snapshot to include tagging across all supported locations including directive definitions.
src/HotChocolate/Core/test/Types.Tests/Types/Directives/snapshots/TagDirectiveTests.CodeFirst_Tag.graphql	Updates snapshot for code-first tagging, including directive-definition tagging.
src/HotChocolate/Core/test/Types.Tests/Types/Directives/snapshots/TagDirectiveTests.ImplementationFirst_Tag.graphql	Adds snapshot for implementation-first tagging, including directive-definition tagging.
src/HotChocolate/Core/src/Types/Types/Directives/TagDirectiveExtensions.cs	Adds Tag(this IDirectiveTypeDescriptor, string) to support tagging directive definitions in code-first.
src/HotChocolate/Core/src/Types/Types/Directives/TagAttribute.cs	Enables [Tag] to configure IDirectiveTypeDescriptor (directive definition tagging in implementation-first).
src/HotChocolate/Core/src/Types/Types/Directives/Tag.cs	Extends @tag directive locations to include DIRECTIVE_DEFINITION (non-federation mode).
src/HotChocolate/Caching/test/Caching.Tests/SchemaTests.cs	Updates expected @tag SDL locations to include DIRECTIVE_DEFINITION.
src/HotChocolate/ApolloFederation/test/ApolloFederation.Tests/Directives/TagDirectiveTests.cs	Adds a federation-mode guard test asserting DIRECTIVE_DEFINITION is excluded.

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[github-actions]

Patch coverage

98.6% of changed lines covered (141/143)

File	Covered	Changed	Patch %
…/src/Fusion.Composition/DirectiveDefinitionCompatibility.cs	56	58	96.6% 🟡
…/HotChocolate/Core/src/Types/Types/Directives/TagAttribute.cs	2	2	100.0% 🟢
…/Core/src/Types/Types/Directives/TagDirectiveExtensions.cs	6	6	100.0% 🟢
…/Definitions/TagMutableDirectiveDefinition.cs	2	2	100.0% 🟢
…/Fusion/src/Fusion.Composition/SourceSchemaMerger.cs	3	3	100.0% 🟢
…/Fusion/src/Fusion.Composition/SourceSchemaPreprocessor.cs	72	72	100.0% 🟢

Uncovered changed lines (JSON)

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  "sha": "dfe30243209cd6df676a74b1089765ebc4802652",
  "files": [
    { "path": "src/HotChocolate/Fusion/src/Fusion.Composition/DirectiveDefinitionCompatibility.cs", "ranges": [[24, 25]] }
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Project coverage: 52.6% (217218/413313 lines)