fix: allow parsing non-file data in multipart form data bodies via echo binding

[DMRobertson]

For your consideration: this is intended to reproduce #1262, see https://github.com/oapi-codegen/oapi-codegen/issues/1262#issuecomment-2868180395 in particular. I've also included a proposed fix which lets you Bind() to read non-file parameters in an multipart/form-data. It looked like getting echo to `Bind() file-type parts would be a more invasive change though.

I'm not sure if this is the right change, or even if you agree that the report in 1262 is valid! But I spent enough time investigating that I thought it was worth sharing.