feat: Add standalone Java SeekToBeginning test to reproduce the issue

Created:
- SeekToBeginningTest.java: Standalone Java test that reproduces the seekToBeginning() hang
- Dockerfile.seektest: Docker setup for running the test
- pom.xml: Maven build configuration
- Updated docker-compose.yml to include seek-test service

This test simulates what Schema Registry does:
1. Create KafkaConsumer connected to gateway
2. Assign to _schemas topic partition 0
3. Call seekToBeginning()
4. Poll for records

Expected behavior: Should send ListOffsets and then Fetch
Actual behavior: Blocks indefinitely after seekToBeginning()

test/kafka/kafka-client-loadtest/SeekToBeginningTest.java

@@ -0,0 +1,73 @@
+import org.apache.kafka.clients.consumer.*;
+import org.apache.kafka.common.TopicPartition;
+import org.apache.kafka.common.serialization.ByteArrayDeserializer;
+import java.util.*;
+
+/**
+ * Test program to reproduce the seekToBeginning() hang issue
+ * 
+ * This simulates what Schema Registry does:
+ * 1. Create KafkaConsumer
+ * 2. Assign to partition
+ * 3. Call seekToBeginning()
+ * 4. Poll for records
+ * 
+ * Expected behavior: Consumer should send ListOffsets and then Fetch requests
+ * Observed behavior: Consumer sends InitProducerId but not ListOffsets, then
+ * hangs
+ */
+public class SeekToBeginningTest {
+    public static void main(String[] args) throws Exception {
+        String bootstrapServers = "localhost:9093";
+        String topicName = "_schemas";
+
+        if (args.length > 0) {
+            bootstrapServers = args[0];
+        }
+
+        Properties props = new Properties();
+        props.put(ConsumerConfig.BOOTSTRAP_SERVERS_CONFIG, bootstrapServers);
+        props.put(ConsumerConfig.GROUP_ID_CONFIG, "test-seek-group");
+        props.put(ConsumerConfig.CLIENT_ID_CONFIG, "test-seek-client");
+        props.put(ConsumerConfig.AUTO_OFFSET_RESET_CONFIG, "earliest");
+        props.put(ConsumerConfig.ENABLE_AUTO_COMMIT_CONFIG, "false");
+        props.put(ConsumerConfig.KEY_DESERIALIZER_CLASS_CONFIG, ByteArrayDeserializer.class);
+        props.put(ConsumerConfig.VALUE_DESERIALIZER_CLASS_CONFIG, ByteArrayDeserializer.class);
+        props.put(ConsumerConfig.SESSION_TIMEOUT_MS_CONFIG, "45000");
+        props.put(ConsumerConfig.REQUEST_TIMEOUT_MS_CONFIG, "60000");
+
+        System.out.println("[TEST] Creating KafkaConsumer connecting to " + bootstrapServers);
+        KafkaConsumer<byte[], byte[]> consumer = new KafkaConsumer<>(props);
+
+        TopicPartition tp = new TopicPartition(topicName, 0);
+        List<TopicPartition> partitions = Arrays.asList(tp);
+
+        System.out.println("[TEST] Assigning to partition: " + tp);
+        consumer.assign(partitions);
+
+        System.out.println("[TEST] Calling seekToBeginning()...");
+        long startTime = System.currentTimeMillis();
+        consumer.seekToBeginning(partitions);
+        long seekTime = System.currentTimeMillis() - startTime;
+        System.out.println("[TEST] seekToBeginning() completed in " + seekTime + "ms");
+
+        System.out.println("[TEST] Starting poll loop (30 second timeout per poll)...");
+        for (int i = 0; i < 3; i++) {
+            System.out.println("[POLL " + (i + 1) + "] Polling for records...");
+            long pollStart = System.currentTimeMillis();
+            ConsumerRecords<byte[], byte[]> records = consumer.poll(java.time.Duration.ofSeconds(30));
+            long pollTime = System.currentTimeMillis() - pollStart;
+            System.out.println("[POLL " + (i + 1) + "] Got " + records.count() + " records in " + pollTime + "ms");
+
+            for (ConsumerRecord<byte[], byte[]> record : records) {
+                System.out.println("  [RECORD] offset=" + record.offset() + ", key.len=" +
+                        (record.key() != null ? record.key().length : 0) +
+                        ", value.len=" + (record.value() != null ? record.value().length : 0));
+            }
+        }
+
+        System.out.println("[TEST] Closing consumer...");
+        consumer.close();
+        System.out.println("[TEST] Done!");
+    }
+}