fix(variable): ensure unique variable names in var-list

[Minamiyama]

Summary

When generating default variable names, check for existing names and append a suffix if needed to prevent duplicates

fixes #22037

Screenshots

Before

befor.auto-naming.mp4

After:

after.auto-naming.mp4

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