Add debug logging to GetTrackDownloadURL

The library returns download_info but GetTrackDownloadURL cannot extract
a working URL (bestInfo.URL is empty). Added debug logging of all
download info candidates and a more descriptive error message to
diagnose why Yandex API returns entries without URL.

This is a ym-dl issue, not a library issue — the library correctly
returns download_info; ym-dl fails to build a download URL from it.

internal/yandex/client.go

@@ -221,6 +221,12 @@ func (c *YandexClient) GetTrackDownloadURL(ctx context.Context, trackID string)
 		return "", fmt.Errorf("no suitable download info found")
 	}
 
+	if c.debug {
+		for i, info := range downloadInfos {
+			c.logger.Debug("download info candidate", "index", i, "codec", info.Codec, "bitrate", info.Bitrate, "direct_link", info.DirectLink, "url", info.URL)
+		}
+	}
+
 	// If we have a direct URL, use it
 	if bestInfo.URL != "" && bestInfo.DirectLink {
 		return bestInfo.URL, nil
@@ -253,5 +259,5 @@ func (c *YandexClient) GetTrackDownloadURL(ctx context.Context, trackID string)
 		return finalURL, nil
 	}
 
-	return "", fmt.Errorf("no download URL available")
+	return "", fmt.Errorf("no download URL available: bestInfo.URL is empty (codec=%s, bitrate=%d, direct_link=%v)", bestInfo.Codec, bestInfo.Bitrate, bestInfo.DirectLink)
 }