refactor: replace hardcoded user plan strings with CloudPlan enum

[laipz8200]

Summary

This PR introduces a CloudPlan StrEnum to replace hardcoded user plan string literals throughout the backend codebase, improving type safety and maintainability.

Changes:

• Created api/enums/cloud_plan.py with CloudPlan enum (SANDBOX, PROFESSIONAL, TEAM)
• Used Python's auto() for automatic lowercase value generation
• Replaced hardcoded 'sandbox' strings with CloudPlan.SANDBOX in 18 production files
• Updated 4 test files for consistency
• All existing tests pass, lint and type checks pass

Benefits:

• Type Safety: Compile-time checking prevents typos and invalid plan values
• Maintainability: Centralized plan definitions in one location
• Code Clarity: Explicit enum members are more self-documenting than string literals
• Consistency: Ensures uniform plan identifiers across the codebase

Fixes #27674

Screenshots

N/A - Backend refactoring only, no UI changes

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 backend codebase by replacing hardcoded user plan string literals with a new CloudPlan StrEnum. This change aims to enhance the application's robustness and readability by centralizing plan definitions, improving type safety, and ensuring consistent usage across various modules. The refactoring covers both production and test files, making the system more maintainable and less prone to errors related to plan identification.

Highlights

• New CloudPlan StrEnum: Introduced a CloudPlan StrEnum (SANDBOX, PROFESSIONAL, TEAM) to centralize user plan definitions, using Python's auto() for automatic lowercase value generation.
• Enhanced Type Safety and Maintainability: Replaced hardcoded 'sandbox' string literals with CloudPlan.SANDBOX across 18 production files and updated 4 test files, significantly improving type safety, code clarity, and maintainability.
• Improved Consistency: Ensures uniform plan identifiers throughout the backend codebase, reducing the likelihood of typos and invalid plan values.

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[gemini-code-assist]

Code Review

This pull request is a solid refactoring that replaces hardcoded plan strings with a CloudPlan enum, significantly improving type safety and maintainability across the codebase. The changes are well-executed and consistent. I've added a couple of suggestions to further leverage the new enum for even better type safety in a Pydantic model and a request parser, which aligns with the goals of this PR.