chore: dep inject for model in datasets document and web human input …

[likalikali]

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Summary

#36659

Replace manual DocumentMetadataUpdatePayload.model_validate(request.get_json() or {}) and HumanInputFormSubmitPayload.model_validate(request.get_json()) with the @model_validate decorator introduced in #36750, injecting the validated payload as a method argument.

Screenshots

N/A — backend-only refactoring, no UI changes.

Checklist

• This change requires a documentation update, included: https://github.com/langgenius/dify-docs
• 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	57.99%	57.99%	+0.00%
Strict coverage	57.56%	57.56%	+0.00%
Typed symbols	37,825	37,827	+2
Untyped symbols	27,605	27,605	0
Modules	3133	3133	0