fix(http): expose structured vars in HTTP body selector

[owldev127]

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Summary

Fixes #34153

Problem

Users could not select non-string structured variables in the HTTP node body editor, even though those values were valid workflow outputs and expected to work in request bodies. This forced manual serialization as a workaround.

The frontend HTTP body editor filtered available variables too aggressively and excluded VarType.object and VarType.arrayObject before the selector tree was built, so those variables never appeared in the picker.

Solution

Allow object and array[object] variables in the HTTP body selector and add a regression test.

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