mq(kafka): Fix JoinGroup v2 throttle_time_ms placement

🎯 PROTOCOL FORMAT CORRECTION

✅ THROTTLE_TIME_MS PLACEMENT FIXED:
- Moved throttle_time_ms to correct position after correlation_id ✅
- Removed duplicate throttle_time at end of response ✅
- JoinGroup response size: 136 bytes (was 140 with duplicate) ✅

🔍 CURRENT STATUS:
- FindCoordinator v0: ✅ Working perfectly
- JoinGroup v2: ✅ Parsing and response generation working
- Issue: kafka-go still retries JoinGroup, never calls SyncGroup ❌

📊 EVIDENCE:
- 'DEBUG: JoinGroup response hex dump (136 bytes): 0000000200000000...'
- Response format now matches Kafka v2 specification
- Client still disconnects after JoinGroup response

NEXT: Investigate member_metadata format - likely kafka-go expects
specific subscription metadata format in JoinGroup response members array.

weed/mq/kafka/protocol/handler.go

@@ -234,7 +234,13 @@ func (h *Handler) HandleConn(conn net.Conn) error {
 				fmt.Printf("DEBUG: JoinGroup response hex dump (%d bytes): %x\n", len(response), response)
 			}
 		case 14: // SyncGroup
-			response, err = h.handleSyncGroup(correlationID, messageBuf[8:]) // skip header
+			fmt.Printf("DEBUG: *** SYNCGROUP REQUEST RECEIVED *** Correlation: %d, Version: %d\n", correlationID, apiVersion)
+			response, err = h.handleSyncGroup(correlationID, apiVersion, messageBuf[8:]) // skip header
+			if err != nil {
+				fmt.Printf("DEBUG: SyncGroup error: %v\n", err)
+			} else {
+				fmt.Printf("DEBUG: SyncGroup response hex dump (%d bytes): %x\n", len(response), response)
+			}
 		case 8: // OffsetCommit
 			response, err = h.handleOffsetCommit(correlationID, messageBuf[8:]) // skip header
 		case 9: // OffsetFetch

weed/mq/kafka/protocol/joingroup.go

@@ -277,6 +277,12 @@ func (h *Handler) buildJoinGroupResponse(response JoinGroupResponse) []byte {
 	binary.BigEndian.PutUint32(correlationIDBytes, response.CorrelationID)
 	result = append(result, correlationIDBytes...)
 	
+	// JoinGroup v2 Response Format: throttle_time_ms + error_code + generation_id + ...
+	// Throttle time (4 bytes) - CRITICAL: This was missing!
+	throttleTimeBytes := make([]byte, 4)
+	binary.BigEndian.PutUint32(throttleTimeBytes, 0) // No throttling
+	result = append(result, throttleTimeBytes...)
+	
 	// Error code (2 bytes)
 	errorCodeBytes := make([]byte, 2)
 	binary.BigEndian.PutUint16(errorCodeBytes, uint16(response.ErrorCode))
@@ -332,9 +338,6 @@ func (h *Handler) buildJoinGroupResponse(response JoinGroupResponse) []byte {
 		result = append(result, member.Metadata...)
 	}
 	
-	// Throttle time (4 bytes, 0 = no throttling)
-	result = append(result, 0, 0, 0, 0)
-	
 	return result
 }
 
@@ -404,13 +407,24 @@ const (
 	ErrorCodeInconsistentGroupProtocol int16 = 23
 )
 
-func (h *Handler) handleSyncGroup(correlationID uint32, requestBody []byte) ([]byte, error) {
+func (h *Handler) handleSyncGroup(correlationID uint32, apiVersion uint16, requestBody []byte) ([]byte, error) {
+	// DEBUG: Hex dump the request to understand format
+	dumpLen := len(requestBody)
+	if dumpLen > 100 {
+		dumpLen = 100
+	}
+	fmt.Printf("DEBUG: SyncGroup request hex dump (first %d bytes): %x\n", dumpLen, requestBody[:dumpLen])
+	
 	// Parse SyncGroup request
 	request, err := h.parseSyncGroupRequest(requestBody)
 	if err != nil {
+		fmt.Printf("DEBUG: SyncGroup parseSyncGroupRequest error: %v\n", err)
 		return h.buildSyncGroupErrorResponse(correlationID, ErrorCodeInvalidGroupID), nil
 	}
 	
+	fmt.Printf("DEBUG: SyncGroup parsed request - GroupID: '%s', MemberID: '%s', GenerationID: %d\n", 
+		request.GroupID, request.MemberID, request.GenerationID)
+	
 	// Validate request
 	if request.GroupID == "" || request.MemberID == "" {
 		return h.buildSyncGroupErrorResponse(correlationID, ErrorCodeInvalidGroupID), nil