Wrap `zip.NewReader` errors in NuGet `ParsePackageMetaData` and
`ExtractPortablePdb` as `ErrInvalidArgument` so invalid packages return
HTTP 400 (Bad Request) instead of 500 (Internal Server Error).
Add integration test for multipart/form-data NuGet upload path (used by
`dotnet nuget push`) which was previously untested.
Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Claude (Opus 4.6) <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>