fix: strip base URL path prefix from helm chart download URLs

When a helm repo's base URL includes a path (e.g. stakater.github.io/
stakater-charts), the merger extracted the full path from download URLs
causing duplication: /stakater-charts/stakater-charts/reloader.tgz.

Now strips the base URL's path prefix from same-host download URLs so
the proxy constructs the correct upstream URL.

internal/virtual/helm_merger.go

@@ -65,11 +65,12 @@ func (m *HelmMerger) MergeIndexes(members []MemberIndex, proxyBaseURL string) ([
 							if baseHost != "" && extractHost(u) != baseHost {
 								continue
 							}
+							relPath := extractPathRelativeToBase(u, member.BaseURL)
 							ver.URLs[i] = fmt.Sprintf("%s/api/v1/%s/%s/%s",
 								strings.TrimRight(proxyBaseURL, "/"),
 								routePrefix,
 								member.RemoteName,
-								extractPath(u))
+								relPath)
 						} else {
 							ver.URLs[i] = fmt.Sprintf("%s/api/v1/%s/%s/%s",
 								strings.TrimRight(proxyBaseURL, "/"),
@@ -102,6 +103,18 @@ func extractHost(rawURL string) string {
 	return rest[:slashIdx]
 }
 
+func extractPathRelativeToBase(rawURL, baseURL string) string {
+	fullPath := extractPath(rawURL)
+	basePath := extractPath(baseURL)
+	if basePath != "" {
+		basePath = strings.TrimRight(basePath, "/") + "/"
+		if strings.HasPrefix(fullPath, basePath) {
+			return fullPath[len(basePath):]
+		}
+	}
+	return fullPath
+}
+
 func extractPath(rawURL string) string {
 	idx := strings.Index(rawURL, "://")
 	if idx == -1 {