fix: guard chat tree against out-of-order parents

[iamjoel]

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 85.34%. Comparing base (6540d17) to head (44ef6f2).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #38590      +/-   ##
==========================================
- Coverage   85.34%   85.34%   -0.01%     
==========================================
  Files        5017     5017              
  Lines      266979   266992      +13     
  Branches    50824    50827       +3     
==========================================
+ Hits       227844   227855      +11     
- Misses      34668    34670       +2     
  Partials     4467     4467              

Flag	Coverage Δ
dify-ui	94.61% <ø> (ø)
web	84.84% <100.00%> (-0.01%)	⬇️

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