feat(extension-repo): preserve package suffix from download URL

Detect .sflx and .spotiflac-ext from the registry download URL when saving
repo downloads, and extract findExtension so the destination path can use
the correct package extension.

go_backend/exports.go

@@ -3916,13 +3916,29 @@ func GetStoreCategoriesJSON() (string, error) {
 	return string(jsonBytes), nil
 }
 
-func buildStoreExtensionDestPath(destDir, extensionID string) (string, error) {
+func storeExtensionPackageSuffix(downloadURL string) string {
+	rawPath := downloadURL
+	if parsed, err := url.Parse(downloadURL); err == nil {
+		rawPath = parsed.Path
+	}
+
+	lowerPath := strings.ToLower(rawPath)
+	if strings.HasSuffix(lowerPath, ".sflx") {
+		return ".sflx"
+	}
+	if strings.HasSuffix(lowerPath, ".spotiflac-ext") {
+		return ".spotiflac-ext"
+	}
+	return ".spotiflac-ext"
+}
+
+func buildStoreExtensionDestPath(destDir, extensionID, downloadURL string) (string, error) {
 	if strings.TrimSpace(extensionID) == "" {
 		return "", fmt.Errorf("invalid extension id")
 	}
 
 	safeExtensionID := sanitizeFilename(extensionID)
-	return filepath.Join(destDir, safeExtensionID+".spotiflac-ext"), nil
+	return filepath.Join(destDir, safeExtensionID+storeExtensionPackageSuffix(downloadURL)), nil
 }
 
 func DownloadStoreExtensionJSON(extensionID, destDir string) (string, error) {
@@ -3931,7 +3947,12 @@ func DownloadStoreExtensionJSON(extensionID, destDir string) (string, error) {
 		return "", fmt.Errorf("extension store not initialized")
 	}
 
-	destPath, err := buildStoreExtensionDestPath(destDir, extensionID)
+	ext, err := store.findExtension(extensionID)
+	if err != nil {
+		return "", err
+	}
+
+	destPath, err := buildStoreExtensionDestPath(destDir, extensionID, ext.getDownloadURL())
 	if err != nil {
 		return "", err
 	}

go_backend/exports_supplement_test.go

@@ -428,10 +428,25 @@ func TestExportsJSONWrappersAndExtensionManagerSurface(t *testing.T) {
 	if catsJSON, err := GetStoreCategoriesJSON(); err != nil || !strings.Contains(catsJSON, "metadata") {
 		t.Fatalf("GetStoreCategoriesJSON = %q/%v", catsJSON, err)
 	}
-	if dest, err := buildStoreExtensionDestPath(dir, "coverage/ext"); err != nil || !strings.HasSuffix(dest, ".spotiflac-ext") {
+	if dest, err := buildStoreExtensionDestPath(
+		dir,
+		"coverage/ext",
+		"https://registry.example.com/coverage.spotiflac-ext",
+	); err != nil || !strings.HasSuffix(dest, ".spotiflac-ext") {
 		t.Fatalf("buildStoreExtensionDestPath = %q/%v", dest, err)
 	}
-	if _, err := buildStoreExtensionDestPath(dir, " "); err == nil {
+	if dest, err := buildStoreExtensionDestPath(
+		dir,
+		"coverage/ext",
+		"https://registry.example.com/coverage.sflx",
+	); err != nil || !strings.HasSuffix(dest, ".sflx") {
+		t.Fatalf("buildStoreExtensionDestPath sflx = %q/%v", dest, err)
+	}
+	if _, err := buildStoreExtensionDestPath(
+		dir,
+		" ",
+		"https://registry.example.com/coverage.sflx",
+	); err == nil {
 		t.Fatal("expected invalid extension id")
 	}
 	if err := ClearStoreCacheJSON(); err != nil {

go_backend/extension_store.go

@@ -330,22 +330,26 @@ func (s *extensionStore) getExtensionsWithStatus(forceRefresh bool) ([]storeExte
 	return result, nil
 }
 
-func (s *extensionStore) downloadExtension(extensionID string, destPath string) error {
+func (s *extensionStore) findExtension(extensionID string) (*storeExtension, error) {
 	registry, err := s.fetchRegistry(false)
 	if err != nil {
-		return err
+		return nil, err
 	}
 
-	var ext *storeExtension
 	for _, e := range registry.Extensions {
 		if e.ID == extensionID {
-			ext = &e
-			break
+			ext := e
+			return &ext, nil
 		}
 	}
 
-	if ext == nil {
-		return fmt.Errorf("extension %s not found in store", extensionID)
+	return nil, fmt.Errorf("extension %s not found in store", extensionID)
+}
+
+func (s *extensionStore) downloadExtension(extensionID string, destPath string) error {
+	ext, err := s.findExtension(extensionID)
+	if err != nil {
+		return err
 	}
 
 	if err := requireHTTPSURL(ext.getDownloadURL(), "extension download"); err != nil {