test(hls): Add unit test for EXT-X-DEFINE variable substitution in download path

Adds a test that verifies variables defined via EXT-X-DEFINE NAME/VALUE
in the multivariant playlist are correctly resolved in media playlist
segment URLs during download. Without the fix in HlsPlaylistParser, this
test fails with unresolved {} in segment URLs.

Author: Prashant Kumar

libraries/exoplayer_hls/src/test/java/androidx/media3/exoplayer/hls/offline/HlsDownloaderTest.java

@@ -301,6 +301,45 @@ public void downloadEncMediaPlaylist() throws Exception {
     assertCachedData(cache, fakeDataSet);
   }
 
+  @Test
+  public void download_withVariableSubstitutionInSegmentUrls_resolvesVariables() throws Exception {
+    // Multivariant playlist defines a variable via EXT-X-DEFINE NAME/VALUE
+    byte[] multivariantPlaylistData =
+        ("#EXTM3U\n"
+                + "#EXT-X-DEFINE:NAME=\"cdnPrefix\",VALUE=\"https://cdn.example.com/\"\n"
+                + "#EXT-X-STREAM-INF:BANDWIDTH=232370,CODECS=\"mp4a.40.2, avc1.4d4015\"\n"
+                + "media_with_vars.m3u8\n")
+            .getBytes(java.nio.charset.StandardCharsets.UTF_8);
+
+    // Media playlist uses the variable in segment URLs without IMPORT
+    byte[] mediaPlaylistData =
+        ("#EXTM3U\n"
+                + "#EXT-X-TARGETDURATION:10\n"
+                + "#EXT-X-VERSION:8\n"
+                + "#EXT-X-MEDIA-SEQUENCE:0\n"
+                + "#EXT-X-PLAYLIST-TYPE:VOD\n"
+                + "#EXT-X-DEFINE:IMPORT=\"cdnPrefix\"\n"
+                + "#EXTINF:9.97667,\n"
+                + "{$cdnPrefix}segment0.ts\n"
+                + "#EXTINF:9.97667,\n"
+                + "{$cdnPrefix}segment1.ts\n"
+                + "#EXT-X-ENDLIST")
+            .getBytes(java.nio.charset.StandardCharsets.UTF_8);
+
+    fakeDataSet =
+        new FakeDataSet()
+            .setData("master_vars.m3u8", multivariantPlaylistData)
+            .setData("media_with_vars.m3u8", mediaPlaylistData)
+            .setRandomData("https://cdn.example.com/segment0.ts", 10)
+            .setRandomData("https://cdn.example.com/segment1.ts", 11);
+
+    HlsDownloader downloader = getHlsDownloader("master_vars.m3u8", getKeys(0));
+    downloader.download(progressListener);
+
+    // Verify segments were downloaded with resolved URLs
+    assertCachedData(cache, fakeDataSet);
+  }
+
   private HlsDownloader getHlsDownloader(String mediaPlaylistUri, List<StreamKey> keys) {
     CacheDataSource.Factory cacheDataSourceFactory =
         new CacheDataSource.Factory()