fix: prevent delete default database

Author: gyang94

fluss-server/src/main/java/org/apache/fluss/server/coordinator/MetadataManager.java

@@ -24,6 +24,7 @@
 import org.apache.fluss.exception.DatabaseNotExistException;
 import org.apache.fluss.exception.FlussRuntimeException;
 import org.apache.fluss.exception.InvalidAlterTableException;
+import org.apache.fluss.exception.InvalidDatabaseException;
 import org.apache.fluss.exception.InvalidPartitionException;
 import org.apache.fluss.exception.LakeTableAlreadyExistException;
 import org.apache.fluss.exception.PartitionAlreadyExistsException;
@@ -314,6 +315,11 @@ public Map<String, PartitionRegistration> listPartitions(
 
     public void dropDatabase(String name, boolean ignoreIfNotExists, boolean cascade)
             throws DatabaseNotExistException, DatabaseNotEmptyException {
+        if (CoordinatorServer.DEFAULT_DATABASE.equals(name)) {
+            throw new UnsupportedOperationException(
+                    "Cannot drop the default database '" + name
+                            + "'. The default database is required for cluster operation.");
+        }
         if (!databaseExists(name)) {
             if (ignoreIfNotExists) {
                 return;