Fix VariableEntityType Bug external-data-tool -> external_data_tool

[GuoNingNing]

Checklist:

Important

Please review the checklist below before submitting your pull request.

• Please open an issue before creating a PR or link to an existing issue
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

Description

This pull request fixes a bug related to a constant definition error that caused external_data_tool to become unavailable. Specifically, the definition of VariableEntityType has been updated from external-data-tool to external_data_tool. This change ensures that the corresponding functionality works correctly and avoids issues caused by naming errors.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update, included: Dify Document
• Improvement, including but not limited to code refactoring, performance optimization, and UI/UX improvement
• Dependency upgrade

Testing Instructions

1. Test that the related functionality now works correctly for external_data_tool.
2. Verify that the constant definition is properly resolved.