[Java][Spring] fix RequestPart/RequestParam handling on multipart request

fix for https://github.com/OpenAPITools/openapi-generator/issues/14295 & improvement for https://github.com/OpenAPITools/openapi-generator/pull/11449

Idea of this fix is pretty simple. As long as request object isModel then generate @RequestPart, otherwise @RequestParam

According to Spring docs mentioned here https://github.com/OpenAPITools/openapi-generator/pull/11449 this should be fine. As long as param is a complex object we have to treat it as JSON/XML part, otherwise as a simple param(string,number,boolean,date)

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