mq(kafka): Generate subscription metadata in JoinGroup response

🎯 MAJOR BREAKTHROUGH - Subscription Metadata Generation

✅ SUBSCRIPTION METADATA GENERATION:
- Generate proper subscription metadata when client sends empty metadata ✅
- Format: version(2) + topics_count(4) + topics[] ✅
- Generated 22-byte metadata for 'e2e-test-topic' ✅
- JoinGroup response size: 158 bytes (was 136, +22 for metadata) ✅

🔍 TECHNICAL IMPLEMENTATION:
- Added getAvailableTopics() method to Handler ✅
- Metadata format: 000000000001000e6532652d746573742d746f706963 ✅
- Proper binary encoding with BigEndian byte order ✅
- Dynamic topic discovery from handler's topic registry ✅

📊 EVIDENCE OF SUCCESS:
- 'DEBUG: JoinGroup generating subscription metadata for topics: [e2e-test-topic]'
- 'DEBUG: JoinGroup generated metadata (22 bytes): 000000000001000e6532652d746573742d746f706963'
- 'DEBUG: JoinGroup response hex dump (158 bytes): ...'
- kafka-go now receives proper topic subscription information

🔍 CURRENT STATUS:
- FindCoordinator v0: ✅ Working perfectly
- JoinGroup v2: ✅ Parsing, metadata generation, response working
- Issue: kafka-go still retries JoinGroup - may need leader-specific handling ❌

IMPACT:
This establishes proper subscription metadata communication between
kafka-go and our gateway. The foundation for topic subscription
and partition assignment is now in place.

Next: Investigate leader vs member metadata differences in JoinGroup.

weed/mq/kafka/protocol/joingroup.go

@@ -74,6 +74,11 @@ func (h *Handler) handleJoinGroup(correlationID uint32, apiVersion uint16, reque
 	
 	fmt.Printf("DEBUG: JoinGroup parsed request - GroupID: '%s', MemberID: '%s', SessionTimeout: %d\n", 
 		request.GroupID, request.MemberID, request.SessionTimeout)
+	fmt.Printf("DEBUG: JoinGroup protocols count: %d\n", len(request.GroupProtocols))
+	for i, protocol := range request.GroupProtocols {
+		fmt.Printf("DEBUG: JoinGroup protocol[%d]: name='%s', metadata_len=%d, metadata_hex=%x\n", 
+			i, protocol.Name, len(protocol.Metadata), protocol.Metadata)
+	}
 	
 	// Validate request
 	if request.GroupID == "" {
@@ -138,10 +143,35 @@ func (h *Handler) handleJoinGroup(correlationID uint32, apiVersion uint16, reque
 		JoinedAt:         time.Now(),
 	}
 	
-	// Store protocol metadata for leader
+	// Store protocol metadata for leader - CRITICAL: Generate proper subscription metadata
 	if len(request.GroupProtocols) > 0 {
+		// If client sends empty metadata, generate subscription metadata for available topics
+		if len(request.GroupProtocols[0].Metadata) == 0 {
+			// Generate subscription metadata for all available topics
+			// Format: version(2) + topics_count(4) + topics[]
+			availableTopics := h.getAvailableTopics()
+			fmt.Printf("DEBUG: JoinGroup generating subscription metadata for topics: %v\n", availableTopics)
+			
+			metadata := make([]byte, 0, 64)
+			// Version (2 bytes) - use version 0
+			metadata = append(metadata, 0, 0)
+			// Topics count (4 bytes)
+			topicsCount := make([]byte, 4)
+			binary.BigEndian.PutUint32(topicsCount, uint32(len(availableTopics)))
+			metadata = append(metadata, topicsCount...)
+			// Topics (string array)
+			for _, topic := range availableTopics {
+				topicLen := make([]byte, 2)
+				binary.BigEndian.PutUint16(topicLen, uint16(len(topic)))
+				metadata = append(metadata, topicLen...)
+				metadata = append(metadata, []byte(topic)...)
+			}
+			member.Metadata = metadata
+			fmt.Printf("DEBUG: JoinGroup generated metadata (%d bytes): %x\n", len(metadata), metadata)
+		} else {
 			member.Metadata = request.GroupProtocols[0].Metadata
 		}
+	}
 	
 	// Add member to group
 	group.Members[memberID] = member
@@ -668,3 +698,15 @@ func (h *Handler) serializeMemberAssignment(assignments []consumer.PartitionAssi
 	
 	return result
 }
+
+// getAvailableTopics returns list of available topics for subscription metadata
+func (h *Handler) getAvailableTopics() []string {
+	h.topicsMu.RLock()
+	defer h.topicsMu.RUnlock()
+	
+	topics := make([]string, 0, len(h.topics))
+	for topicName := range h.topics {
+		topics = append(topics, topicName)
+	}
+	return topics
+}