refactor(variables): clarify base vs union type naming

[laipz8200]

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Summary

Refactors variable type naming so the abstract base class is VariableBase and the Pydantic union is Variable, updating imports and type hints across workflow/runtime/services/tests for clarity.
Fixes #30632

Screenshots

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Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

[gemini-code-assist]

Summary of Changes

Hello @laipz8200, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request refactors the core variable type definitions to improve clarity and maintainability. By renaming the base class to VariableBase and the Pydantic union type to Variable, the distinction between the abstract definition and the concrete, serializable representation of variables is made explicit. This change propagates throughout the application, updating all references to these types for consistency.

Highlights

• Type Renaming: The base class for variables, previously named Variable, has been renamed to VariableBase. This change clarifies its role as an abstract base class for all specific variable types.
• Union Type Simplification: The VariableUnion type alias, which represents a union of all concrete variable types and is primarily used for Pydantic serialization and deserialization, has been renamed to Variable. This makes the type name more intuitive and aligns with common usage patterns.
• Codebase-wide Consistency: All relevant files across the codebase have been updated to reflect these new naming conventions. This includes changes to imports, type hints, isinstance checks, and comments, ensuring a consistent and clear variable type system throughout the project.

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[gemini-code-assist]

Code Review

This pull request is a large-scale refactoring that renames Variable to VariableBase and VariableUnion to Variable to improve type name clarity. The changes are applied consistently across the entire codebase, including application logic, workflow components, models, and tests. My review confirms that the renaming has been executed thoroughly. I've identified one minor opportunity to improve a comment for better code clarity and maintainability. Overall, this is a solid refactoring that enhances the readability of the code.