Refactor to use StructuredMessageDecoder directly instead of StatefulStructuredMessageDecoder

- Removed StatefulStructuredMessageDecoder class as requested
- Modified StorageContentValidationDecoderPolicy to use StructuredMessageDecoder directly with DecoderState wrapper
- DecoderState class now properly tracks encoder/decoder positions for smart retries
- Maintains all smart retry functionality:
  * Incremental segment-by-segment decoding using decoder.decode(buffer, size)
  * Checksum validation as complete segments are received
  * State preservation across network interruptions via context
  * Proper offset tracking for both encoded and decoded streams

The implementation ensures that when network faults occur mid-segment:
- Partial segment data is buffered in pendingBuffer
- Retry continues from exact interruption point in encoded stream
- Complete segments are validated with their checksums
- No re-downloading of already received data

Co-authored-by: gunjansingh-msft <179915668+gunjansingh-msft@users.noreply.github.com>

Author: Copilot

sdk/storage/azure-storage-blob/src/main/java/com/azure/storage/blob/specialized/BlobAsyncClientBase.java

@@ -1403,7 +1403,7 @@ Mono<BlobDownloadAsyncResponse> downloadStreamWithResponse(BlobRange range, Down
                             Object decoderState = firstRangeContext.getData(Constants.STRUCTURED_MESSAGE_DECODER_STATE_CONTEXT_KEY)
                                 .orElse(null);
                             if (decoderState != null) {
-                                retryContext = firstRangeContext.addData(Constants.STRUCTURED_MESSAGE_DECODER_STATE_CONTEXT_KEY, decoderState);
+                                retryContext = retryContext.addData(Constants.STRUCTURED_MESSAGE_DECODER_STATE_CONTEXT_KEY, decoderState);
                             }
                         }