fix: restore divided-commitlog fallback in DLedgerCommitLog#getMaxOffset

getCommittedPos() returns a negative value when no entry has been
committed yet, which is exactly the state of a freshly started DLedger
store layered on top of legacy commitlog data. The `committedPos == 0`
guard therefore skipped the dLedgerFileList.getMinOffset() fallback that
was previously applied for any non-positive committed position, so
getMaxOffset() and getConfirmOffset() returned 0 instead of the divided
commitlog offset. DefaultMessageStore then computed a negative
dispatchBehindBytes() and left reputFromOffset behind, failing
MixCommitlogTest#testPutAndGet with expected:<0> but was:<-1163232>.

Also make StoreTestBase#nextPort() probe that a candidate port can
actually be bound before handing it out. The counter previously returned
port numbers blindly, so a stale listener in the 30000+ range made
callers fail with "Address already in use"; the new three-node DLedger
test needs three listeners at once and hit this on CI.

store/src/main/java/org/apache/rocketmq/store/dledger/DLedgerCommitLog.java

@@ -162,7 +162,11 @@ public class DLedgerCommitLog extends CommitLog {
         if (committedPos > 0) {
             return committedPos;
         }
-        if (committedPos == 0 && dLedgerFileList.getMinOffset() > 0) {
+        // getCommittedPos() yields a non-positive value when no entry has been committed yet,
+        // which is the normal state right after a DLedger store is layered on top of legacy
+        // commitlog data. In that case the divided commitlog offset, i.e. the min offset of the
+        // DLedger file list, is the real max offset.
+        if (dLedgerFileList.getMinOffset() > 0) {
             return dLedgerFileList.getMinOffset();
         }
         return 0;

store/src/test/java/org/apache/rocketmq/store/StoreTestBase.java

@@ -24,8 +24,10 @@ import org.apache.rocketmq.common.message.MessageExtBrokerInner;
 import org.junit.After;
 
 import java.io.File;
+import java.io.IOException;
 import java.net.InetAddress;
 import java.net.InetSocketAddress;
+import java.net.ServerSocket;
 import java.net.SocketAddress;
 import java.net.UnknownHostException;
 import java.util.ArrayList;
@@ -47,8 +49,27 @@ public class StoreTestBase {
 
     private static AtomicInteger port = new AtomicInteger(30000);
 
+    private static final int MAX_PORT_PROBE_ATTEMPTS = 200;
+
     public static synchronized int nextPort() {
-        return port.addAndGet(5);
+        for (int i = 0; i < MAX_PORT_PROBE_ATTEMPTS; i++) {
+            int candidate = port.addAndGet(5);
+            if (isPortAvailable(candidate)) {
+                return candidate;
+            }
+        }
+        throw new IllegalStateException("Failed to find an available port after "
+            + MAX_PORT_PROBE_ATTEMPTS + " attempts, last tried " + port.get());
+    }
+
+    private static boolean isPortAvailable(int candidate) {
+        try (ServerSocket serverSocket = new ServerSocket()) {
+            serverSocket.setReuseAddress(false);
+            serverSocket.bind(new InetSocketAddress(candidate));
+            return true;
+        } catch (IOException e) {
+            return false;
+        }
     }
 
     protected MessageExtBatch buildBatchMessage(int size) {