fix: Reduce manual commit frequency from every 10 to every 100 messages

PROBLEM:
Consumer throughput still 45.46 msgs/sec vs producer 50.29 msgs/sec (10% gap).

ROOT CAUSE:
Manual session.Commit() every 10 messages creates excessive overhead:
  - 1,880 messages consumed → 188 commit operations
  - Each commit is SYNCHRONOUS and blocks message processing
  - Auto-commit is already enabled (5s interval)
  - Double-committing reduces effective throughput

ANALYSIS:
  - Test showed consumer lag at 0 at end (not falling behind)
  - Only ~1,880 of 12,200 messages consumed during 2-minute window
  - Consumers start 2s late, need ~262s to consume all at current rate
  - Commit overhead: 188 RPC round trips = significant latency

FIX:
Reduce manual commit frequency from every 10 to every 100 messages:
  - Only 18-20 manual commits during entire test
  - Auto-commit handles primary offset persistence (5s interval)
  - Manual commits serve as backup for edge cases
  - Unblocks message processing loop for higher throughput

EXPECTED IMPACT:
  - Consumer throughput: 45.46 → ~49+ msgs/sec (match producer!)
  - Latency reduction: Fewer synchronous commits
  - Test duration: Should consume all messages before test ends

test/kafka/kafka-client-loadtest/internal/consumer/consumer.go

@@ -647,9 +647,10 @@ func (h *ConsumerGroupHandler) ConsumeClaim(session sarama.ConsumerGroupSession,
 				// Mark message as processed
 				session.MarkMessage(message, "")
 
-				// Commit offset immediately every 10 messages to reduce duplicate window
-				// This supplements auto-commit and ensures offsets are committed more frequently
-				if msgCount%10 == 0 {
+				// Commit offset less frequently to reduce overhead
+				// Auto-commit is enabled (5s interval by default), so manual commits are just backups
+				// Reduced from every 10 messages to every 100 messages to match throughput
+				if msgCount%100 == 0 {
 					session.Commit()
 				}
 			}