Set notice receiver before libpq connection startup

[Chao Li]

Hi,

While testing “Log remote NOTICE, WARNING, and similar messages using 
ereport()”, I noticed that libpqsrv_notice_receiver is only installed after 
libpqsrv_connect() finishes. As a result, NOTICE messages generated during 
connection establishment are missed by ereport() and are still printed to 
stderr.

To reproduce the issue, I created a separate database called remotedb and 
defined a login trigger that emits a NOTICE message:
```
CREATE DATABASE remotedb;

\c remotedb

CREATE OR REPLACE FUNCTION repro_login_notice()
RETURNS event_trigger
LANGUAGE plpgsql AS $$
BEGIN
  RAISE NOTICE 'startup notice from remotedb login trigger';
END;
$$;

CREATE EVENT TRIGGER repro_login_notice_trg
ON login
EXECUTE FUNCTION repro_login_notice();

ALTER EVENT TRIGGER repro_login_notice_trg ENABLE ALWAYS;
```

Then, from another database:
```
evantest=# create extension dblink;
CREATE EXTENSION
evantest=# SELECT dblink_connect('host=127.0.0.1 port=5432 dbname=remotedb 
user=chaol sslmode=disable gssencmode=disable');
 dblink_connect
----------------
 OK
(1 row)
```

In the system log, the NOTICE message is printed directly:
```
2026-05-20 13:02:19.350 CST [24909] STATEMENT:  SELECT 
dblink_connect('host=127.0.0.1 port=5432 dbname=remotedb user=chaol 
sslmode=disable gssencmode=disable');
NOTICE:  startup notice from remotedb login trigger
```

To fix that, I think we should install libpqsrv_notice_receiver before 
libpqsrv_connect_internal(). In the attached patch, I added two helpers: 
libpqsrv_connect_with_notice_receiver() and 
libpqsrv_connect_params_with_notice_receiver().

With the fix, the NOTICE message now looks like this:
```
2026-05-20 14:44:49.296 CST [45567] LOG:  received message via remote 
connection: NOTICE:  startup notice from remotedb login trigger
2026-05-20 14:44:49.296 CST [45567] STATEMENT:  SELECT 
dblink_connect('host=127.0.0.1 port=5432 dbname=remotedb user=chaol 
sslmode=disable gssencmode=disable');
```

Please see the attached patch for details.

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/

v1-0001-Set-notice-receiver-before-libpq-connection-start.patch
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