- TypeScript: Added seenExtends Set and validateExtends method
- Python: Added _visited_extends set and updated _validate_extends_keyword
- Go: Added seenExtends map and validateExtends function
- .NET: Added _visitedExtends HashSet and updated ValidateExtendsKeyword
- Java: Added visitedExtends Set and validateExtendsRef method

Added SCHEMA_EXTENDS_CIRCULAR error code to all SDKs for consistent
error reporting when circular  chains are detected.

Added adversarial test suite with 33 schemas and 22 instances to
stress-test validators for edge cases including:
- Circular references ( and )
- Deep nesting and recursion
- ReDoS patterns
- Unicode edge cases
- Type composition conflicts
- Numeric precision issues