fix: strip base URL path prefix from helm chart download URLs (#64)

When a helm repo base URL includes a path component (e.g. \`stakater.github.io/stakater-charts\`), the merger was extracting the full URL path (\`stakater-charts/reloader-2.2.8.tgz\`) and the proxy then constructed \`base_url/stakater-charts/reloader-2.2.8.tgz\` = double path = 404.

Fix: \`extractPathRelativeToBase()\` strips the shared base path prefix so only the filename portion is used as the proxy path.
Reviewed-on: #64
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

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 {