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Description

Rename the base Variable class to VariableBase and rename VariableUnion to Variable to make the type names match their actual roles (base class vs. Pydantic union for serialization). Update type hints and imports accordingly to improve readability.

Motivation

The current naming makes it easy to confuse the abstract base class with the union type used for serialization. Clarifying the names reduces ambiguity in type hints and helps prevent misuse in runtime checks.

Additional Context

No behavior change expected; this is a naming/type clarity refactor.