[Bug] Failed to load data with singlenode mode using `insert into ... select ...`

### Apache Cloudberry version

_No response_

### What happened

```
postgres=# CREATE TABLE t1 (c1 int) with (appendonly=true, orientation=column);
CREATE TABLE
postgres=# CREATE EXTERNAL TABLE ext_t1 (like t1)
postgres-# LOCATION ('file://hashdata/home/gpadmin/data/data/tpcds10s/t1.dat')
postgres-# FORMAT 'TEXT' (DELIMITER '|' NULL AS '' ESCAPE AS E'\\');
CREATE EXTERNAL TABLE
postgres=# insert into t1 select * from ext_t1;
ERROR: 'ON COORDINATOR' is not supported by this protocol yet
```

### What you think should happen instead

_No response_

### How to reproduce

```
postgres=# CREATE TABLE t1 (c1 int) with (appendonly=true, orientation=column);
CREATE TABLE
postgres=# CREATE EXTERNAL TABLE ext_t1 (like t1)
postgres-# LOCATION ('file://hashdata/home/gpadmin/data/data/tpcds10s/t1.dat')
postgres-# FORMAT 'TEXT' (DELIMITER '|' NULL AS '' ESCAPE AS E'\\');
CREATE EXTERNAL TABLE
postgres=# insert into t1 select * from ext_t1;
ERROR: 'ON COORDINATOR' is not supported by this protocol yet
```

### Operating System

Linux

### Anything else

_No response_

### Are you willing to submit PR?

- [ ] Yes, I am willing to submit a PR!

### Code of Conduct

- [x] I agree to follow this project's [Code of Conduct](https://github.com/apache/cloudberry/blob/main/CODE_OF_CONDUCT.md).

Start by running the provided SQL reproduction in singlenode mode and trace the external-table INSERT ... SELECT entry point that emits the "ON COORDINATOR" error. The issue names no source files or tests, so identify the relevant database execution and external-table tests before confirming the intended successful behavior with maintainers.

Written by the indexing model from the issue text.