Fix: external table regress allow demo text resolver to handle multiple fields

[MisterRaindrop]

---

Change logs

• adjust DemoTextResolver to serialize multi-column records by joining fields
• preserve legacy single-field end-of-stream semantics
• update DemoResolverTest expectation to cover multi-field output

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[MisterRaindrop]

Hi, can anybody review it? @tuhaihe @Mulily0513 @ostinru

[tuhaihe]

Thanks! The related external table test passed. LGTM. Wait for others' comments from a technical view.

[Mulily0513]

LGTM

[ostinru]

Adding context - what we are fixing here. Before this patch there were following error in cloudberry-pxf/external-table/results/pxf.out

------------------------------------------------------------------
-- PXF write test
------------------------------------------------------------------
\!rm -rf /tmp/pxf
INSERT INTO pxf_write_test SELECT * from origin;
ERROR:  PXF server error : Unexpected record format, expected 1 field, found 2  (seg0 172.19.0.3:7002 pid=23746)
HINT:  Check the PXF logs located in the '/home/gpadmin/pxf-base/logs' directory on host 'localhost' or 'set client_min_messages=LOG' for additional details.

And PXF logs from /home/gpadmin/pxf-base/logs/pxf-service.log

java.lang.Exception: Unexpected record format, expected 1 field, found 2
        at org.greenplum.pxf.api.examples.DemoTextResolver.setFields(DemoTextResolver.java:63) ~[pxf-api-1.0.0.jar!/:?]
        at org.greenplum.pxf.service.bridge.WriteBridge.setNext(WriteBridge.java:91) ~[classes!/:1.0.0]
        at org.greenplum.pxf.service.controller.WriteServiceImpl.readStream(WriteServiceImpl.java:67) ~[classes!/:1.0.0]
        at org.greenplum.pxf.service.controller.WriteServiceImpl.lambda$writeData$0(WriteServiceImpl.java:40) ~[classes!/:1.0.0]
        at java.security.AccessController.doPrivileged(Native Method) ~[?:?]
        at javax.security.auth.Subject.doAs(Subject.java:361) ~[?:?]

[ostinru]

LGTM

[tuhaihe]

Cool! Then let me merge this PR. Thanks all guys!