PG18: fix query results diff in merge regress test.

The test used SQL functions which can be cached in PG18+ since
commit 0dca5d68d. This means that a Citus distributed plan can
be cached, but the copy function for a distributed plan did not
include the `sourceResultRepartitionColumnIndex` field, which is
critical for MERGE queries. Including it fixes the diff problem.
Also update `OutDistributedPlan()` function so that the field is
picked up by `nodeToString()`.

src/backend/distributed/utils/citus_copyfuncs.c

@@ -137,6 +137,8 @@ CopyNodeDistributedPlan(COPYFUNC_ARGS)
 	COPY_SCALAR_FIELD(fastPathRouterPlan);
 	COPY_SCALAR_FIELD(numberOfTimesExecuted);
 	COPY_NODE_FIELD(planningError);
+
+	COPY_SCALAR_FIELD(sourceResultRepartitionColumnIndex);
 }
 
 

src/backend/distributed/utils/citus_outfuncs.c

@@ -203,6 +203,7 @@ OutDistributedPlan(OUTFUNC_ARGS)
 	WRITE_UINT_FIELD(numberOfTimesExecuted);
 
 	WRITE_NODE_FIELD(planningError);
+	WRITE_INT_FIELD(sourceResultRepartitionColumnIndex);
 }