feat: add multi-select input variable type

[ddddajie]

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Related Issue

Related to langgenius/dify#40575

Summary

Add a new MULTI_SELECT member to VariableEntityType so consumers can represent user inputs that allow selecting multiple predefined values.

This is a prerequisite for supporting native multi-select / checkbox-group inputs in Dify Start Nodes.

The change only introduces the input variable type. Existing Graphon array[string] runtime support is reused, so no new segment type or runtime behavior is introduced.

Changes

• Add VariableEntityType.MULTI_SELECT = "multi-select"
• Add validation coverage for multi-select VariableEntity
• Add serialization coverage to ensure the wire value remains "multi-select"

Tests

Targeted tests:

tests/variables/test_input_entities.py

Result:

14 passed

Also verified:

• VariableEntity.model_validate() accepts "multi-select"
• model_dump(mode="json") serializes the type as "multi-select"
• existing SegmentType.ARRAY_STRING support handles typed empty arrays

Checklist

• This pull request links the issue it advances
• I have added tests for the change
• The change is backward compatible

[github-actions]

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[ddddajie]

I have read the CLA Document and I hereby sign the CLA

[ddddajie]

Hi @WH-2099 , could you please take a look at this PR when you have a chance?

This is a small prerequisite for langgenius/dify#40575. It only adds VariableEntityType.MULTI_SELECT while reusing the existing ARRAY_STRING runtime type, with focused validation/serialization coverage (2 files, +25 lines; 14 targeted tests passing; CLA signed).

The Dify-side implementation is already prepared and is currently waiting on this Graphon prerequisite before final integration and the upstream Dify PR.

Happy to make any changes you think are needed. Thanks!