feat: @oneOf input directive support

[samuelAndalon]

📝 Description

Add support for GraphQL @oneOf directive for input objects .

See:
https://graphql.org/blog/2025-09-04-multioption-inputs-with-oneof/
https://spec.graphql.org/September2025/#sec-OneOf-Input-Objects

What graphql-java provides

graphql-java owns the directive definition and all input validation— runs before our convertArgumentValue:

• Directive def — @OneOf SDL definition baked into graphql-java.

• Input validation/coercion — ValuesResolverOneOfValidation (source) enforces oneOf rules at execution time, per spec:

• exactly one field supplied,

• that field's value non-null,

• rejects map with 0 or 2+ keys -> error before resolver runs.

  So by the time graphql-kotlin runtime code sees the argument map, it is already validated: guaranteed single non-null entry. That's why mapToOneOfKotlinObject can safely
  do input.entries.single() — no defensive count check needed.

What graphql-java does NOT cover

graphql-java works in graphql land. It knows nothing about Kotlin sealed types. It does not:

• generate the @OneOf input object from Kotlin types
• know which sealed subtype a given field maps to
• construct the Kotlin instance at runtime during operation execution

What graphql-kotlin adds

1. Schema generation (generateOneOfInputObject / generateOneOfInputProperty)
   • Kotlin sealed interface -> @OneOf input object.
   • Each subtype (@GraphQLOneOfField) -> one input field
2. Argument mapping (mapToOneOfKotlinObject in convertArgumentValue)
   • Takes the already-validated single-entry map from graphql-java.
   • Matches fieldName -> sealed subtype via @GraphQLOneOfField.

🔗 Related Issues

#1891

Examples