Move non-repeatable directive uniqueness to schema validation

[glen-84]

Summary

• Types.Validation gains DirectiveIsUniqueRule (HCV0031), which reports a non-repeatable directive applied more than once at the same location, one entry per directive name however many times it repeats.
• The Mutable SchemaParser no longer throws when an extension re-applies a non-repeatable directive. MergeDirectives is removed and all three extension paths append like every other path, so the invalid state is representable and reported by validation instead of aborting the parse.
• SchemaValidator publishes directive events for the schema definition, so SCHEMA-location applications reach the directive rules. Apollo Federation source schemas apply @link to the schema without declaring it, so a @link definition is registered for them; RemoveFederationInfrastructure already strips both the definition and its applications during preprocessing.

Test plan

• DirectiveIsUniqueRuleTests covers every location the validator traverses in one case — schema, type, output field, argument, input field, enum value, directive definition, and directive-definition argument — plus a repeatable directive, three applications collapsing to one entry, the type-extension path, and a spec directive.
• The three SchemaParserTests extension cases assert both applications are kept instead of asserting a throw. The argument path had no coverage before.
• Full suites run for Types.Validation (141), Types.Mutable (88), and Fusion.Composition (740), plus Types.Abstractions, Adapters.OpenApi, Fusion.Utilities, and Fusion.Execution as the remaining consumers of Types.Validation and the Mutable parser.

[Copilot]

Pull request overview

Moves enforcement of non-repeatable directive uniqueness from the mutable schema parse/merge phase into the central schema validation pipeline, allowing invalid (but representable) schema states to be parsed and then surfaced consistently as validation errors.

Changes:

• Added DirectiveIsUniqueRule (HCV0031) and wired it into SchemaValidator, including publishing directive events for the schema definition.
• Updated the mutable SchemaParser extension paths to append directives (instead of throwing) and adjusted parser tests accordingly.
• Improved Fusion Federation source parsing by registering an explicit @link directive definition so schema-level applications bind and can be validated/processed consistently.

Reviewed changes

Copilot reviewed 12 out of 14 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
website/content/docs/fusion/source-schema-extensions.md	Updates docs to reflect parser now keeps duplicate non-repeatable directives and relies on validation to fail composition.
src/HotChocolate/Mutable/test/Types.Mutable.Tests/SchemaParserTests.cs	Updates/extends tests to assert duplicates are retained for extension scenarios (field, argument, directive definition).
src/HotChocolate/Mutable/src/Types.Mutable/Serialization/SchemaParser.cs	Removes directive merge-time uniqueness throwing and switches extension paths to append directives.
src/HotChocolate/Mutable/src/Types.Mutable/Properties/MutableResources.resx	Removes the no-longer-used parser error string for duplicate non-repeatable directives.
src/HotChocolate/Mutable/src/Types.Mutable/Properties/MutableResources.Designer.cs	Removes the generated accessor for the deleted parser resource string.
src/HotChocolate/Fusion/src/Fusion.Composition/SourceSchemaParser.cs	Registers @link for federation sources and keeps @requires registration conditional; supports schema-level directive validation.
src/HotChocolate/Fusion/src/Fusion.Composition/Definitions/LinkMutableDirectiveDefinition.cs	Adds a minimal mutable directive definition for Apollo Federation @link (schema location, repeatable).
src/HotChocolate/Core/test/Types.Validation.Tests/Rules/DirectiveIsUniqueRuleTests.cs	Adds coverage for DirectiveIsUniqueRule across all traversed locations and repeatable/spec cases.
src/HotChocolate/Core/src/Types.Validation/SchemaValidator.cs	Adds DirectiveIsUniqueRule to default rules and publishes directive events for the schema definition.
src/HotChocolate/Core/src/Types.Validation/Rules/DirectiveIsUniqueRule.cs	Implements the non-repeatable directive uniqueness validation rule.
src/HotChocolate/Core/src/Types.Validation/Properties/ValidationResources.resx	Adds the localized message template for the new uniqueness validation error.
src/HotChocolate/Core/src/Types.Validation/Properties/ValidationResources.Designer.cs	Adds the generated accessor for the new validation resource string.
src/HotChocolate/Core/src/Types.Validation/Logging/LogEntryHelper.cs	Adds DirectiveNotUnique log entry builder and unifies directive member naming for related messages.
src/HotChocolate/Core/src/Types.Validation/Logging/LogEntryCodes.cs	Adds HCV0031 code constant for directive uniqueness violations.

Files not reviewed (2)

• src/HotChocolate/Core/src/Types.Validation/Properties/ValidationResources.Designer.cs: Generated file
• src/HotChocolate/Mutable/src/Types.Mutable/Properties/MutableResources.Designer.cs: Generated file

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[github-actions]

Patch coverage

98.7% of changed lines covered (74/75)

File	Covered	Changed	Patch %
…/Core/src/Types.Validation/Logging/LogEntryHelper.cs	19	20	95.0% 🟡
…/Core/src/Types.Validation/Rules/DirectiveIsUniqueRule.cs	21	21	100.0% 🟢
…/HotChocolate/Core/src/Types.Validation/SchemaValidator.cs	2	2	100.0% 🟢
…/Definitions/LinkMutableDirectiveDefinition.cs	21	21	100.0% 🟢
…/Fusion/src/Fusion.Composition/SourceSchemaParser.cs	8	8	100.0% 🟢
…/Mutable/src/Types.Mutable/Serialization/SchemaParser.cs	3	3	100.0% 🟢

Uncovered changed lines (JSON)

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  "sha": "f1f20e92964ab665297e3ec9f272580aa169ce90",
  "files": [
    { "path": "src/HotChocolate/Core/src/Types.Validation/Logging/LogEntryHelper.cs", "ranges": [[641, 641]] }
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}

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Project coverage: 54.4% (242365/445717 lines)