[fix](fe) Fix NLJ build-side source_deps crash when serial NLJ gets PASSTHROUGH

### What problem does this PR solve?

Issue Number: close #xxx

Problem Summary: When NLJ is serial (RIGHT_OUTER/ANTI/SEMI/FULL_OUTER),
shouldResetSerialFlagForChild(1) was unconditionally true, clearing the
serial ancestor flag for the build side. This caused Exchange(UNPARTITIONED)
on the build side to insert a PASSTHROUGH local exchange, restoring the
build pipeline's num_tasks to _num_instances while the probe pipeline stays
at 1. The extra build tasks had NLJ shared states with empty source_deps,
crashing in Dependency::set_ready_to_read().

Fix: shouldResetSerialFlagForChild(1) now returns false when NLJ is serial,
preserving the serial ancestor flag on the build side so Exchange returns
NOOP instead of inserting PASSTHROUGH.

### Release note

None

### Check List (For Author)

- Test: Regression test added (Bug 19 in test_local_shuffle_rqg_bugs.groovy)
- Behavior changed: No
- Does this need documentation: No

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: 924060929

fe/fe-core/src/main/java/org/apache/doris/planner/NestedLoopJoinNode.java

@@ -231,6 +231,14 @@ public Pair<PlanNode, LocalExchangeType> enforceAndDeriveLocalExchange(PlanTrans
 
     @Override
     protected boolean shouldResetSerialFlagForChild(int childIndex) {
-        return childIndex == 1;
+        // Build side (child 1) is a separate pipeline in BE.  Normally,
+        // the serial-ancestor flag should be reset across pipeline boundaries.
+        // BUT when NLJ itself is serial (RIGHT_OUTER/ANTI/SEMI/FULL_OUTER),
+        // the probe pipeline has num_tasks=1.  If we reset the flag, the
+        // build-side Exchange may insert PASSTHROUGH (restoring num_tasks to
+        // _num_instances), creating more build tasks than probe tasks.  The
+        // extra build tasks have a NLJ shared state with empty source_deps,
+        // crashing in set_ready_to_read().
+        return childIndex == 1 && !isSerialOperator();
     }
 }

regression-test/suites/nereids_p0/local_shuffle/test_local_shuffle_rqg_bugs.groovy

@@ -1041,5 +1041,55 @@ suite("test_local_shuffle_rqg_bugs") {
         assertTrue(false, "Bug 18: Serial NLJ PASSTHROUGH LE crash: ${t.message}")
     }
 
+    //  Bug 19: source_deps.size()=0 crash in NLJ build sink.
+    //  Root cause: serial NLJ (RIGHT_OUTER) resets serial ancestor flag for build side.
+    //  Exchange(UNPARTITIONED) on build side sees hasSerialAncestorInPipeline=false and
+    //  inserts PASSTHROUGH LE. This restores build pipeline num_tasks to _num_instances
+    //  while probe pipeline stays at 1. The extra build tasks have NLJ shared state with
+    //  empty source_deps → crash in set_ready_to_read().
+    //  Fix: shouldResetSerialFlagForChild(1) returns false when NLJ is serial.
+    //  Differs from Bug 18 in fuzzy vars: enable_share_hash_table=true, broadcast_passthrough=false.
+    try {
+        logger.info("Bug 19: Testing serial NLJ build-side source_deps crash")
+        def bug19_baseline = sql """
+            SELECT /*+SET_VAR(enable_local_shuffle_planner=false,
+                              enable_sql_cache=false)*/
+                table1.col_varchar_10__undef_signed AS field1
+            FROM rqg_t1 AS table1
+            LEFT JOIN rqg_t1 AS table2
+                ON table2.pk = table2.pk
+            WHERE table1.pk BETWEEN 2 AND 11
+            GROUP BY field1
+            ORDER BY 1
+        """
+
+        for (int ppt : [2, 4]) {
+            def bug19_result = sql """
+                SELECT /*+SET_VAR(use_serial_exchange=false,
+                                  parallel_pipeline_task_num=${ppt},
+                                  enable_local_shuffle_planner=true,
+                                  ignore_storage_data_distribution=true,
+                                  enable_sql_cache=false,
+                                  enable_share_hash_table_for_broadcast_join=true,
+                                  enable_broadcast_join_force_passthrough=false,
+                                  enable_parallel_scan=true,
+                                  disable_streaming_preaggregations=true)*/
+                    table1.col_varchar_10__undef_signed AS field1
+                FROM rqg_t1 AS table1
+                LEFT JOIN rqg_t1 AS table2
+                    ON table2.pk = table2.pk
+                WHERE table1.pk BETWEEN 2 AND 11
+                GROUP BY field1
+                ORDER BY 1
+            """
+            assertEquals(bug19_baseline, bug19_result,
+                "Bug 19 pptn=${ppt}: result mismatch with serial NLJ build side crash")
+        }
+        logger.info("Bug 19: PASSED (no crash, correct results)")
+    } catch (Throwable t) {
+        logger.error("Bug 19 FAILED: ${t.message}")
+        assertTrue(false, "Bug 19: Serial NLJ build-side source_deps crash: ${t.message}")
+    }
+
     logger.info("=== All RQG bug reproduction tests completed ===")
 }