Create sub-issue for issue #688: Implement OneOf Input Objects (Input Unions)

Author: pavelnikolov

issues/688/sub-issues/Implement-OneOf-Input-Objects.md

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+# Implement OneOf Input Objects (Input Unions)
+
+Implement support for OneOf input objects as specified in the September 2025 GraphQL spec.
+
+**Reference:** 
+- Parent issue: #688
+- Spec PR: https://github.com/graphql/graphql-spec/pull/825
+
+**Description:**
+OneOf input objects, also known as 'input unions', allow exactly one field to be specified in an input object. This provides a way to model union-like behavior for input types.
+
+**Implementation Requirements:**
+- [ ] Add OneOf directive support in schema parsing
+- [ ] Implement validation rules for OneOf input objects
+- [ ] Ensure exactly one field is provided in OneOf inputs
+- [ ] Add introspection support for OneOf
+- [ ] Update documentation and examples
+
+**Related to:** Target Sep 2025 GraphQL spec implementation