[duplicated subtitle][YouTube] Add helper methods for parsing YouTube subtitle URL parameters.

- Add `V`, `LANG`, `TLANG` constants to `YoutubeParsingHelper`
- Implement `extractVideoId()`, `extractLanguageCode()`, `extractTranslationCode()`
- Add `extractQueryParam()` utility in `Utils.java`

Author: TransZAllen

extractor/src/main/java/org/schabi/newpipe/extractor/services/youtube/YoutubeParsingHelper.java

@@ -130,6 +130,13 @@ private YoutubeParsingHelper() {
     public static final String CPN = "cpn";
     public static final String VIDEO_ID = "videoId";
 
+    /**
+     * YouTube-specific URL query parameters
+     */
+    public static final String V = "v";          // Video ID parameter in URL (e.g., v=VIDEO_ID)
+    public static final String LANG = "lang";    // Original language parameter (e.g., lang=en)
+    public static final String TLANG = "tlang";  // Auto-translate language (e.g., tlang=zh-CN)
+
     /**
      * A parameter sent by official clients named {@code contentCheckOk}.
      *
@@ -194,6 +201,22 @@ private YoutubeParsingHelper() {
 
     private static boolean consentAccepted = false;
 
+    // Extract the videoId (e.g., "b7vmW_5HSpE") from a YouTube subtitle URL
+    // (e.g., .../api/timedtext?v=b7vmW_5HSpE)
+    public static String extractVideoId(final String urlString) {
+        return Utils.extractQueryParam(urlString, V);
+    }
+
+    // Extracts the original language code (e.g., 'en') from a YouTube URL.
+    public static String extractLanguageCode(final String urlString) {
+        return Utils.extractQueryParam(urlString, LANG);
+    }
+
+    // Extracts the 'Auto-translate' language code from a YouTube URL.
+    public static String extractTranslationCode(final String urlString) {
+        return Utils.extractQueryParam(urlString, TLANG);
+    }
+
     public static boolean isGoogleURL(final String url) {
         final String cachedUrl = extractCachedUrlIfNeeded(url);
         try {

extractor/src/main/java/org/schabi/newpipe/extractor/utils/Utils.java

@@ -195,6 +195,24 @@ public static URL stringToURL(final String url) throws MalformedURLException {
         }
     }
 
+    /**
+     * Extracts the value of a query parameter from a URL string.
+     *
+     * @param urlString the URL to parse
+     * @param key the query parameter key (e.g., "v", "lang", "tlang")
+     * @return the parameter value if present, otherwise {@code null}
+     */
+    public static String extractQueryParam(final String urlString,
+                                           final String key) {
+        try {
+            final URL url = stringToURL(urlString);
+            return getQueryValue(url, key);
+        } catch (final Exception e) {
+            // Invalid or malformed URL, or unexpected parsing error
+            return null;
+        }
+    }
+
     public static boolean isHTTP(@Nonnull final URL url) {
         // Make sure it's HTTP or HTTPS
         final String protocol = url.getProtocol();