mq(kafka): Investigate SyncGroup workflow - kafka-go not calling SyncGroup

🔍 CRITICAL FINDINGS - Consumer Group Protocol Analysis

✅ CONFIRMED WORKING:
- FindCoordinator API (key 10) ✅
- JoinGroup API (key 11) ✅
- Deterministic member ID generation ✅
- No more JoinGroup retries ✅

❌ CONFIRMED NOT WORKING:
- SyncGroup API (key 14) - NEVER called by kafka-go ❌
- Fetch API (key 1) - NEVER called by kafka-go ❌

🔍 OBSERVED BEHAVIOR:
- kafka-go calls: FindCoordinator → JoinGroup → (stops)
- kafka-go makes repeated Metadata requests
- No progression to SyncGroup or Fetch
- Test fails with 'context deadline exceeded'

🎯 HYPOTHESIS:
kafka-go may be:
1. Using simplified consumer protocol (no SyncGroup)
2. Expecting specific JoinGroup response format
3. Waiting for specific error codes/state transitions
4. Using different rebalancing strategy

📊 EVIDENCE:
- JoinGroup response: 215 bytes, includes member metadata
- Group state: Empty → PreparingRebalance → CompletingRebalance
- Member ID: consistent across calls (4b60f587)
- Protocol: 'range' selection working

NEXT: Research kafka-go consumer group implementation
to understand why SyncGroup is bypassed.

weed/mq/kafka/protocol/handler.go

@@ -235,6 +235,7 @@ func (h *Handler) HandleConn(conn net.Conn) error {
 			}
 		case 14: // SyncGroup
 			fmt.Printf("DEBUG: *** SYNCGROUP REQUEST RECEIVED *** Correlation: %d, Version: %d\n", correlationID, apiVersion)
+			fmt.Printf("DEBUG: *** THIS IS CRITICAL - SYNCGROUP WAS CALLED! ***\n")
 			response, err = h.handleSyncGroup(correlationID, apiVersion, messageBuf[8:]) // skip header
 			if err != nil {
 				fmt.Printf("DEBUG: SyncGroup error: %v\n", err)

weed/mq/kafka/protocol/joingroup.go

@@ -149,8 +149,13 @@ func (h *Handler) handleJoinGroup(correlationID uint32, apiVersion uint16, reque
 			group.Generation++
 			fmt.Printf("DEBUG: JoinGroup transitioned to PreparingRebalance, new generation: %d\n", group.Generation)
 		}
-	case consumer.GroupStatePreparingRebalance, consumer.GroupStateCompletingRebalance:
-		// Rebalance already in progress
+	case consumer.GroupStatePreparingRebalance:
+		// Rebalance in progress - if this is the leader and we have members, transition to CompletingRebalance
+		if len(group.Members) > 0 && memberID == group.Leader {
+			group.State = consumer.GroupStateCompletingRebalance
+			fmt.Printf("DEBUG: JoinGroup leader '%s' transitioning group to CompletingRebalance (ready for SyncGroup)\n", memberID)
+		}
+	case consumer.GroupStateCompletingRebalance:
 		// Allow join but don't change generation until SyncGroup
 	case consumer.GroupStateDead:
 		return h.buildJoinGroupErrorResponse(correlationID, ErrorCodeInvalidGroupID), nil