Don't use 2PC if COPY operates on a single shard

See discussion in:
https://github.com/citusdata/citus/pull/7160#discussion_r1314488889

src/backend/distributed/commands/multi_copy.c

@@ -2048,8 +2048,12 @@ CitusCopyDestReceiverStartup(DestReceiver *dest, int operation,
 
 	UseCoordinatedTransaction();
 
-	/* all modifications use 2PC */
+	/* all modifications use 2PC unless we only touch a single shard */
+	if (!IsCitusTableTypeCacheEntry(cacheEntry, SINGLE_SHARD_DISTRIBUTED) &&
+		!IsCitusTableTypeCacheEntry(cacheEntry, CITUS_LOCAL_TABLE))
+	{
 		Use2PCForCoordinatedTransaction();
+	}
 
 	/* define how tuples will be serialised */
 	CopyOutState copyOutState = (CopyOutState) palloc0(sizeof(CopyOutStateData));

src/test/regress/expected/foreign_tables_mx.out

@@ -38,9 +38,16 @@ SELECT partmethod, repmodel FROM pg_dist_partition WHERE logicalrelid = 'foreign
  n          | s
 (1 row)
 
--- COPY FROM doesn't work for Citus foreign tables
+-- COPY FROM for Citus foreign tables
 COPY foreign_table FROM stdin;
-ERROR:  cannot PREPARE a transaction that has operated on postgres_fdw foreign tables
+SELECT * FROM foreign_table ORDER BY a;
+ id |   data    | a
+---------------------------------------------------------------------
+  1 | text_test | 1
+  1 | 1foo      | 2
+(2 rows)
+
+DELETE FROM foreign_table WHERE a = 2;
 CREATE TABLE parent_for_foreign_tables (
     project_id integer
 ) PARTITION BY HASH (project_id);

src/test/regress/sql/foreign_tables_mx.sql

@@ -37,11 +37,14 @@ CREATE FOREIGN TABLE foreign_table (
 --verify
 SELECT partmethod, repmodel FROM pg_dist_partition WHERE logicalrelid = 'foreign_table'::regclass ORDER BY logicalrelid;
 
--- COPY FROM doesn't work for Citus foreign tables
+-- COPY FROM for Citus foreign tables
 COPY foreign_table FROM stdin;
 1	1foo	2
 \.
 
+SELECT * FROM foreign_table ORDER BY a;
+DELETE FROM foreign_table WHERE a = 2;
+
 CREATE TABLE parent_for_foreign_tables (
     project_id integer
 ) PARTITION BY HASH (project_id);