fix: fix LOOP_CHILDREN_Z_INDEX

[fatelei]

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Summary

fix #30712

fix copy node z-index is not right

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 make lint and make type-check (backend) and cd web && npx lint-staged (frontend) to appease the lint gods

[gemini-code-assist]

Summary of Changes

Hello @fatelei, 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 addresses an issue where copied nodes, particularly those pasted into Loop blocks, exhibited incorrect z-index stacking, leading to display problems. The changes ensure that pasted nodes maintain their proper visual layering and that their parent-child relationships are accurately established within the workflow editor, improving the consistency and reliability of node interactions.

Highlights

• Corrected Z-Index for Copied Loop Children: When nodes are copied and pasted into a Loop block, their zIndex is now correctly set to LOOP_CHILDREN_Z_INDEX, resolving display order issues where nodes might appear incorrectly layered.
• Improved Parent-Child Relationship Handling on Paste: The logic for pasting nodes into Iteration and Loop blocks has been enhanced to correctly update the parent node's _children array, ensuring that the parent node properly recognizes its newly pasted children.
• Accurate Parent ID Assignment: The iteration_id and loop_id for pasted child nodes are now correctly assigned using the id of the parent selectedNode, rather than potentially incorrect data properties, ensuring proper nesting.
• Consistent Node Properties for New Loop Children: New nodes generated within a loop now consistently receive _dimmed: false, isInLoop: true, loop_id, and type properties, ensuring their correct behavior and appearance within the loop context.

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[gemini-code-assist]

Code Review

This pull request correctly fixes an issue where the z-index of a copied node was not being set correctly when pasted into a Loop block. The changes look good overall. I've added one comment with a suggestion to refactor a section of the code. This refactoring would not only reduce duplication but also fix a related bug where the z-index was missed for nodes pasted into Iteration blocks, and another potential bug related to inconsistent node state when pasting between different container types.