feat(graphql): support one of input objects

packages/graphql/lib/decorators/input-type.decorator.ts

@@ -25,6 +25,11 @@ export interface InputTypeOptions {
    * If `true`, type will not be registered in the schema.
    */
   isAbstract?: boolean;
+  /**
+   * If 'true', the input type will be '@oneOf' type.
+   * More info about '@oneOf' types in the [GraphQL spec](https://spec.graphql.org/September2025/#sec-OneOf-Input-Objects).
+   */
+  isOneOf?: boolean;
 }
 
 /**
@@ -67,6 +72,7 @@ export function InputType(
       name: name || target.name,
       description: options.description,
       isAbstract: options.isAbstract,
+      isOneOf: options.isOneOf,
     };
     LazyMetadataStorage.store(() =>
       TypeMetadataStorage.addInputTypeMetadata(metadata),

packages/graphql/lib/schema-builder/factories/input-type-definition.factory.ts

@@ -34,6 +34,7 @@ export class InputTypeDefinitionFactory {
       type: new GraphQLInputObjectType({
         name: metadata.name,
         description: metadata.description,
+        isOneOf: metadata.isOneOf,
         fields: this.generateFields(metadata, options),
         /**
          * AST node has to be manually created in order to define directives

packages/graphql/lib/schema-builder/metadata/class.metadata.ts

@@ -10,4 +10,5 @@ export interface ClassMetadata {
   extensions?: Record<string, unknown>;
   properties?: PropertyMetadata[];
   inheritDescription?: boolean;
+  isOneOf?: boolean; // For '@oneOf' input types
 }