fix: knowledge hit-testing render failed. (#36106)

[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.Fixes #36090

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

Screenshots

Before	After
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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 && 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%	43.19%	+43.19%
Typed symbols	0	22,043	+22,043
Untyped symbols	0	28,756	+28,756
Modules	0	2552	+2,552