Set requiredness for primitive and files in forms

[captainsafia]

Closes #57112.

This PR updates the handling of required parameters within the form body to match the framework implementation.

• The top-level OpenApiRequestBody.Required property is always set to true for forms. The framework does not support empty bodies for form-based requests in the same way that it does for JSON-based requests.
• The OpenApiRequestBody.Content.Required property is updated to include required parameters of a primitive type in the list if they are required.
• While testing this I discovered Empty form body with optional form file-based argument produces unexpected 408 in minimal APIs #57195, which makes it difficult to verify the optional of FormFile/FormFileCollection. Requiredness is still configured for these parameter types but we'll need to fix the underlying bug and test the end-to-end.
• Requiredness for complex types (like [FromForm] Todo todo) is not affected by this change since the JsonSchemaExporter will set requiredness correctly for complex types.

[mikekistler]

Looks good. 👍