Use the unsafe future for read single region (#109139)

We need to use the unsafe future to block on the same thread pool
similar to read multi regions.

x-pack/plugin/blob-cache/src/main/java/org/elasticsearch/blobcache/shared/SharedBlobCacheService.java

@@ -1116,7 +1116,9 @@ public class SharedBlobCacheService<KeyType> implements Releasable {
             RangeMissingHandler writer,
             int region
         ) throws InterruptedException, ExecutionException {
-            final PlainActionFuture<Integer> readFuture = new PlainActionFuture<>();
+            final PlainActionFuture<Integer> readFuture = new UnsafePlainActionFuture<>(
+                BlobStoreRepository.STATELESS_SHARD_PREWARMING_THREAD_NAME
+            );
             final CacheFileRegion fileRegion = get(cacheKey, length, region);
             final long regionStart = getRegionStart(region);
             fileRegion.populateAndRead(