Fix decoder duplicate processing by tracking consumed bytes correctly

The root cause was that after calling decoder.decode(), we were saving the ENTIRE combined buffer (pending + new) to the pending buffer, including bytes already consumed by the decoder. This caused the decoder to see duplicate segment headers on subsequent iterations, leading to "Unexpected segment number" errors.

The fix:
1. Changed decoder.decode() call from using dataToProcess.duplicate() to using dataToProcess directly
2. Track how many bytes were consumed by comparing buffer size before and after decode
3. Only save UNCONSUMED bytes to the pending buffer
4. This ensures the decoder receives a continuous, non-duplicate stream of bytes

Example flow:
- Iteration 1: pending=null, new=[bytes 0-4], combine=[bytes 0-4], decoder consumes 0 (not enough), pending=[bytes 0-4]
- Iteration 2: pending=[bytes 0-4], new=[byte 5], combine=[bytes 0-5], decoder consumes 0 (not enough), pending=[bytes 0-5]
- ...
- Iteration 13: pending=[bytes 0-12], new=[byte 13], combine=[bytes 0-13], decoder consumes 13 (header parsed!), pending=null
- Iteration 14: pending=null, new=[byte 14], decoder continues from where it left off

Addresses comments #2499104452 and #3447938815.

Co-authored-by: gunjansingh-msft <[email protected]>

Author: Copilot

sdk/storage/azure-storage-common/src/main/java/com/azure/storage/common/policy/StorageContentValidationDecoderPolicy.java

@@ -93,15 +93,23 @@ private Flux<ByteBuffer> decodeStream(Flux<ByteBuffer> encodedFlux, DecoderState
             state.totalEncodedBytesProcessed.addAndGet(encodedBytesInBuffer);
 
             try {
+                // Record the initial size to calculate how many bytes the decoder consumed
+                int initialSize = dataToProcess.remaining();
+                
                 // Try to decode what we have - decoder handles partial data
+                // Pass the buffer directly (not a duplicate) so we can track consumption
                 int availableSize = dataToProcess.remaining();
-                ByteBuffer decodedData = state.decoder.decode(dataToProcess.duplicate(), availableSize);
+                ByteBuffer decodedData = state.decoder.decode(dataToProcess, availableSize);
+
+                // Calculate how many bytes were consumed by the decoder
+                int bytesConsumed = initialSize - dataToProcess.remaining();
 
                 // Track decoded bytes
                 int decodedBytes = decodedData.remaining();
                 state.totalBytesDecoded.addAndGet(decodedBytes);
 
-                // Store any remaining unprocessed data for next iteration
+                // Store any remaining UNCONSUMED data for next iteration
+                // Only save bytes that the decoder hasn't processed yet
                 if (dataToProcess.hasRemaining()) {
                     state.updatePendingBuffer(dataToProcess);
                 } else {