Improve nuget/rubygems package registries (#34741)

1. Add some missing (optional) fields for nuget v2, and sort the fields
to make it easier to maintain
2. Add missing "platform" for rubygems: `VERSION-PLATFORM` and
`VERSION_PLATFORM`

Co-authored-by: Giteabot <teabot@gitea.io>

routers/api/packages/rubygems/rubygems_test.go

@@ -0,0 +1,41 @@
+// Copyright 2025 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package rubygems
+
+import (
+	"strings"
+	"testing"
+
+	rubygems_module "code.gitea.io/gitea/modules/packages/rubygems"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestWritePackageVersion(t *testing.T) {
+	buf := &strings.Builder{}
+
+	writePackageVersionForList(nil, "1.0", " ", buf)
+	assert.Equal(t, "1.0 ", buf.String())
+	buf.Reset()
+
+	writePackageVersionForList(&rubygems_module.Metadata{Platform: "ruby"}, "1.0", " ", buf)
+	assert.Equal(t, "1.0 ", buf.String())
+	buf.Reset()
+
+	writePackageVersionForList(&rubygems_module.Metadata{Platform: "linux"}, "1.0", " ", buf)
+	assert.Equal(t, "1.0_linux ", buf.String())
+	buf.Reset()
+
+	writePackageVersionForDependency("1.0", "", buf)
+	assert.Equal(t, "1.0 ", buf.String())
+	buf.Reset()
+
+	writePackageVersionForDependency("1.0", "ruby", buf)
+	assert.Equal(t, "1.0 ", buf.String())
+	buf.Reset()
+
+	writePackageVersionForDependency("1.0", "os", buf)
+	assert.Equal(t, "1.0-os ", buf.String())
+	buf.Reset()
+}