fix(distributed): drain last operator output in PipelineDriver to prevent infinite loop

The processOnce() loop only passed output between adjacent operator pairs
(i to i+1), never calling getOutput() on the last operator. Operators that
buffer pages (e.g., PassThroughOperator) would never have their buffer
drained, causing isFinished() to never return true and an infinite loop
in run().

Author: vamsimanohar

core/src/main/java/org/opensearch/sql/planner/distributed/pipeline/PipelineDriver.java

@@ -164,6 +164,18 @@ boolean processOnce() {
       }
     }
 
+    // Drain the last operator's output so it can transition to finished.
+    // Without this, operators that buffer pages (e.g., PassThroughOperator)
+    // would never have getOutput() called, preventing isFinished() from
+    // returning true.
+    if (!operators.isEmpty()) {
+      Operator last = operators.get(operators.size() - 1);
+      Page output = last.getOutput();
+      if (output != null) {
+        madeProgress = true;
+      }
+    }
+
     return madeProgress;
   }