[fix](local shuffle) Fix COREDUMP: skip LE insertion when child is serial in enforceChild

When a streaming AGG requests PASSTHROUGH from a serial UNPARTITIONED
Exchange child, enforceChild would insert a PASSTHROUGH local exchange
creating a pipeline split. This disconnects the AggSink↔AggSource shared
state pairing because the LE boundary puts them in separate pipelines
with mismatched task counts, causing source_deps to be empty → DCHECK
(ASAN) or SIGSEGV (Release) in set_ready_to_read().

Fix: add childOutput.first.isSerialOperator() check in enforceChild,
mirroring BE's need_to_local_exchange which checks any_of(operators[idx..end],
is_serial) to skip LE insertion when a serial operator is in the pipeline.

Reproducer: multi-distinct COUNT query on table with many buckets
(non-pooling) where MultiCastDataSinks feed serial UNPARTITIONED
Exchanges → streaming AGG fragments.

Author: 924060929

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

@@ -979,7 +979,13 @@ protected Pair<PlanNode, LocalExchangeType> enforceChild(
         Pair<PlanNode, LocalExchangeType> childOutput = deriveAndEnforceChildLocalExchange(
                 translatorContext, originChild, requireChild, childIndex);
         if (!requireChild.satisfy(childOutput.second)) {
-            if (translatorContext.hasSerialAncestorInPipeline(this) || isSerialOperator()) {
+            // Mirrors BE need_to_local_exchange: any_of(operators[idx..end], is_serial) → skip.
+            // Check ancestors (hasSerialAncestorInPipeline), self (isSerialOperator), AND child.
+            // When the child is serial (e.g. UNPARTITIONED Exchange), inserting a PASSTHROUGH LE
+            // creates a pipeline split where AggSink↔AggSource shared state becomes disconnected
+            // because the LE boundary puts them in separate pipelines with mismatched task counts.
+            if (translatorContext.hasSerialAncestorInPipeline(this) || isSerialOperator()
+                    || childOutput.first.isSerialOperator()) {
                 return childOutput;
             }
             LocalExchangeType preferType = AddLocalExchange.resolveExchangeType(