feat: add downloadable release management for desktop client

- Extend DesktopClientReleaseCheckResponse with platform, arch, and channel fields.
- Introduce DesktopClientDownloadableRelease and DesktopClientDownloadableReleaseListResponse interfaces for managing downloadable releases.
- Implement ListDownloadable method in DesktopClientReleaseService to retrieve downloadable releases based on the specified channel.
- Update DesktopClientReleaseCheckResult to include platform, arch, and channel information.
- Add resolveInstallerDownloadURL method to handle download URL resolution for releases.
- Create new endpoint in DesktopReleaseHandler for listing downloadable releases.
- Update router to include the new downloadable releases route.
- Enhance tests to cover the new functionality for resolving download URLs and listing downloadable releases.

server/internal/ops/app/desktop_client_release_test.go

@@ -1,6 +1,8 @@
 package app
 
 import (
+	"context"
+	"errors"
 	"testing"
 	"time"
 
@@ -192,3 +194,56 @@ func TestBuildDesktopClientReleaseObjectKeyReusesContentAddress(t *testing.T) {
 	require.Equal(t, first, second)
 	require.Equal(t, "desktop-client/releases/blobs/"+sum+".exe", first)
 }
+
+func TestResolveDownloadURLResultUsesSignedDownloadURL(t *testing.T) {
+	t.Parallel()
+
+	objectKey := "desktop-client/releases/blobs/client.zip"
+	storage := &desktopReleaseDownloadURLStorage{}
+	svc := &DesktopClientReleaseService{storage: storage}
+
+	result, err := svc.resolveDownloadURLResult(&domain.DesktopClientRelease{
+		DownloadSource: DesktopClientDownloadSourceOSS,
+		OSSObjectKey:   &objectKey,
+	})
+
+	require.NoError(t, err)
+	require.Equal(t, "https://oss.example.test/signed/client.zip?Expires=123", result.DownloadURL)
+	require.Equal(t, []string{objectKey}, storage.downloadURLCalls)
+	require.Empty(t, storage.publicURLCalls)
+}
+
+type desktopReleaseDownloadURLStorage struct {
+	publicURLCalls   []string
+	downloadURLCalls []string
+}
+
+func (s *desktopReleaseDownloadURLStorage) Validate() error {
+	return nil
+}
+
+func (s *desktopReleaseDownloadURLStorage) PutBytes(context.Context, string, []byte, string) error {
+	return nil
+}
+
+func (s *desktopReleaseDownloadURLStorage) GetBytes(context.Context, string) ([]byte, error) {
+	return nil, errors.New("not implemented")
+}
+
+func (s *desktopReleaseDownloadURLStorage) Exists(context.Context, string) (bool, error) {
+	return true, nil
+}
+
+func (s *desktopReleaseDownloadURLStorage) Delete(context.Context, string) error {
+	return nil
+}
+
+func (s *desktopReleaseDownloadURLStorage) PublicURL(objectKey string) (string, error) {
+	s.publicURLCalls = append(s.publicURLCalls, objectKey)
+	return "", errors.New("public url should not be used for private release downloads")
+}
+
+func (s *desktopReleaseDownloadURLStorage) DownloadURL(objectKey string) (string, error) {
+	s.downloadURLCalls = append(s.downloadURLCalls, objectKey)
+	return "https://oss.example.test/signed/client.zip?Expires=123", nil
+}