Fix ExtraFS Breaking SharedClusterSnapshotRestoreIT (#58026) (#58041)

If `ExtraFS` decides to put `extra0/0` into the indices folder
then the previous logic in this test would have interpreted the `0`
as shard `0` of index `extra0` and fail to list its contents (since it's a file
and not an actual shard directory).

=> simplified the logic to use actually referenced `IndexId` for iterating over indices
instead.

server/src/internalClusterTest/java/org/elasticsearch/snapshots/SharedClusterSnapshotRestoreIT.java

@@ -94,7 +94,6 @@
 import org.elasticsearch.snapshots.mockstore.MockRepository;
 import org.elasticsearch.threadpool.ThreadPool;
 
-import java.io.IOException;
 import java.nio.channels.SeekableByteChannel;
 import java.nio.file.Files;
 import java.nio.file.Path;
@@ -1527,20 +1526,14 @@ public void testSnapshotWithMissingShardLevelIndexFile() throws Exception {
             .setWaitForCompletion(true).setIndices("test-idx-*").get();
 
         logger.info("--> deleting shard level index file");
-        try (Stream<Path> files = Files.list(repo.resolve("indices"))) {
-            files.forEach(indexPath -> {
-                try {
-                    final Path shardGen;
-                    try (Stream<Path> shardFiles = Files.list(indexPath.resolve("0"))) {
-                        shardGen = shardFiles
-                            .filter(file -> file.getFileName().toString().startsWith(BlobStoreRepository.INDEX_FILE_PREFIX))
-                            .findFirst().orElseThrow(() -> new AssertionError("Failed to find shard index blob"));
-                    }
-                    Files.delete(shardGen);
-                } catch (IOException e) {
-                    throw new RuntimeException("Failed to delete expected file", e);
-                }
-            });
+        final Path indicesPath = repo.resolve("indices");
+        for (IndexId indexId : getRepositoryData("test-repo").getIndices().values()) {
+            final Path shardGen;
+            try (Stream<Path> shardFiles = Files.list(indicesPath.resolve(indexId.getId()).resolve("0"))) {
+                shardGen = shardFiles.filter(file -> file.getFileName().toString().startsWith(BlobStoreRepository.INDEX_FILE_PREFIX))
+                        .findFirst().orElseThrow(() -> new AssertionError("Failed to find shard index blob"));
+            }
+            Files.delete(shardGen);
         }
 
         logger.info("--> creating another snapshot");