add DefaultBinder negative-path tests (multipart & non-struct)

[miladev95]

Summary

Add three focused unit tests to cover negative/error branches in the binder: TestIsFieldMultipartFile_NonPointer_FileHeader_ReturnsError TestSetMultipartFileHeaderTypes_NoFiles_ReturnsFalse TestDefaultBinder_bindData_NonStruct_ReturnsError

improve test coverage for multipart binding and non-struct binding error handling.