Refactor list parts in GCS

[guy-har]

          > We should have one func to parse the filename into name and number, instead of isPart and extract part number funcs.

@nopcoder I started combining them, but then I noticed that by changing the return value of listMultipartUploadParts to be block.ListPartsResponse it will make things even better. But I prefer not to touch it all while providing the quick fix. I'm opening an issue to refactor, and leaving this as is if it's fine with you

Originally posted by @guy-har in https://github.com/treeverse/lakeFS/issues/9019#issuecomment-2853709030