refactor: migrate web human_input_form from reqparse to Pydantic BaseModel

[ai-hpc]

Fixes #28015

Summary

Replace reqparse.RequestParser with Pydantic BaseModel in the web human input form submission endpoint. Adds HumanInputFormSubmitPayload model with inputs and action fields, aligning with the codebase's ongoing migration to Pydantic for request validation.

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

[github-actions]

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