Fix compilation error in JdbcExecutor.executeSinkStage

The executeSinkStage method introduced in #750 declared filterTasks and
joinTasks as Collection<JdbcFilterOperator> and Collection<JdbcJoinOperator<?>>
respectively, but passed them to createSqlString() which expects
Collection<JdbcExecutionOperator>. Java generics are invariant so this
caused a compilation failure. Fixed by widening both local variable
declarations to Collection<JdbcExecutionOperator>, matching the existing
pattern in the adjacent executeQueryStage method.

Author: gepa

wayang-platforms/wayang-jdbc-template/src/main/java/org/apache/wayang/jdbc/execution/JdbcExecutor.java

@@ -129,9 +129,9 @@ private static void executeSinkStage(final ExecutionStage stage,
         // Extract operators from the stage
         final JdbcTableSource tableOp = (JdbcTableSource) startTask.getOperator();
         final JdbcTableSinkOperator sinkOp = (JdbcTableSinkOperator) termTask.getOperator();
-        final Collection<JdbcFilterOperator> filterTasks = new ArrayList<>(4);
+        final Collection<JdbcExecutionOperator> filterTasks = new ArrayList<>(4);
         JdbcProjectionOperator projectionTask = null;
-        final Collection<JdbcJoinOperator<?>> joinTasks = new ArrayList<>();
+        final Collection<JdbcExecutionOperator> joinTasks = new ArrayList<>();
 
         // Walk through intermediate operators, stopping at the sink
         ExecutionTask nextTask = JdbcExecutor.findJdbcExecutionOperatorTaskInStage(startTask, stage);