增加对 file_fdw 扩展的删除限制，与文档保持一致

src/gausskernel/optimizer/commands/dropcmds.cpp

@@ -116,6 +116,11 @@ static void DropExtensionInListIsSupported(List* objname)
         }
     }
 
+    if (pg_strcasecmp(name, "file_fdw") == 0 && !u_sess->attr.attr_common.IsInplaceUpgrade) {
+        ereport(ERROR, (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+            errmsg("EXTENSION file_fdw does not allow to drop.")));
+    }
+
     /* Enable DROP operation of the above objects during inplace upgrade or support_extended_features is true */
     if (!u_sess->attr.attr_common.IsInplaceUpgrade && !g_instance.attr.attr_common.support_extended_features) {
         ereport(ERROR, (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("EXTENSION is not yet supported.")));

src/test/regress/expected/drop_if_exists.out

@@ -233,3 +233,8 @@ ERROR:  table "test_exists" does not exist
 -- drop the database
 DROP DATABASE if exists abc123_123ABCxar;
 NOTICE:  database "abc123_123abcxar" does not exist, skipping
+-- drop extension
+drop extension file_fdw;
+ERROR:  EXTENSION file_fdw does not allow to drop.
+drop extension file_fdw cascade;
+ERROR:  EXTENSION file_fdw does not allow to drop.

src/test/regress/sql/drop_if_exists.sql

@@ -230,3 +230,7 @@ DROP TABLE test_exists;
 
 -- drop the database
 DROP DATABASE if exists abc123_123ABCxar;
+
+-- drop extension
+drop extension file_fdw;
+drop extension file_fdw cascade;