refactor: enhance node handle components with opacity transitions and add tests for visibility behavior

[CodingOnStar]

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Summary

refactor: enhance node handle components with opacity transitions and add tests for visibility behavior

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.57%. Comparing base (4b6803b) to head (a74e5bb).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #35494      +/-   ##
==========================================
+ Coverage   85.55%   85.57%   +0.02%     
==========================================
  Files        4439     4439              
  Lines      207607   207614       +7     
  Branches    38785    38791       +6     
==========================================
+ Hits       177615   177670      +55     
+ Misses      26876    26828      -48     
  Partials     3116     3116              

Flag	Coverage Δ
dify-ui	93.01% <ø> (ø)
web	86.50% <100.00%> (+0.05%)	⬆️

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