[Fusion] Coerce single element value for list variable

[tobias-tengler]

Closes #9749

[Copilot]

Pull request overview

This PR updates Fusion’s JSON variable coercion to follow the GraphQL spec’s List Input Coercion rule by allowing a non-list JSON value to be treated as a single-element list when the variable type is a list (fixing the behavior reported in #9749).

Changes:

• Update Fusion list-variable coercion to wrap non-array JSON values into a single-item ListValueNode.
• Add Fusion tests covering single-value-to-list coercion and related error cases.
• Add a matching core execution test asserting the error shape when a single value cannot be coerced to the list element type.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/HotChocolate/Fusion/src/Fusion.Execution/Execution/JsonVariableCoercion.cs	Implements single-value → single-item-list coercion for list-typed variables.
src/HotChocolate/Fusion/test/Fusion.Execution.Tests/Execution/VariableCoercionHelperTests.cs	Adds Fusion test coverage for list coercion and error reporting.
src/HotChocolate/Core/test/Execution.Tests/Processing/VariableCoercionHelperTests.cs	Adds core test coverage for the error produced when the single value doesn’t match the list element type.

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