fix: knowledge hit-testing render failed.

[FFXN]

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Summary

Fixes #36090

When hit-testing, an empty document dict is returned due to DocumentSegment type modification.
Then the frontend render failed.

Screenshots

Before	After
...	...

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

[github-actions]

Pyrefly Type Coverage

Metric	Base	PR	Delta
Type coverage	0.00%	43.66%	+43.66%
Strict coverage	0.00%	43.18%	+43.18%
Typed symbols	0	22,044	+22,044
Untyped symbols	0	28,761	+28,761
Modules	0	2552	+2,552

[codecov]

Codecov Report

❌ Patch coverage is 96.87500% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 85.87%. Comparing base (03861bc) to head (b8a594d).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
api/services/hit_testing_service.py	96.42%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #36106   +/-   ##
=======================================
  Coverage   85.87%   85.87%           
=======================================
  Files        4456     4456           
  Lines      210959   210985   +26     
  Branches    39449    39454    +5     
=======================================
+ Hits       181162   181191   +29     
+ Misses      26573    26572    -1     
+ Partials     3224     3222    -2     

Flag	Coverage Δ
api	85.32% <96.87%> (+<0.01%)	⬆️

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