Fix variadic argument types for pg_get_xxx_ddl() functions

Hi,

While testing "[76e514ebb] Add pg_get_role_ddl() function" as well as the other pg_get_xxx_ddl() functions, I noticed that their pg_proc.dat definitions use "text" rather than "_text" for variadic argument types. For example:
```
{ oid => '6501', descr => 'get DDL to recreate a role',
proname => 'pg_get_role_ddl', prorows => '10', provariadic => 'text',
proisstrict => 'f', proretset => 't', provolatile => 's',
pronargdefaults => '1', prorettype => 'text', proargtypes => 'regrole text',
proallargtypes => '{regrole,text}', proargmodes => '{i,v}',
proargdefaults => '{NULL}', prosrc => 'pg_get_role_ddl' },
```

I spotted this problem some time ago, but I wasn’t sure whether it was a real issue worth fixing. When I saw a new patch [1] for pg_get_table_ddl() that had the same problem, I asked about it there. Akshay confirmed the problem and pointed me to the opr_sanity check. Thank you very much, Akshay.

Then I checked why the sanity check didn’t report the issue. I found that the check used != for the comparison. When the final argument type is not an array type, the subquery returns NULL, and NULL != provariadic evaluates to NULL rather than true. I fixed the check to use IS DISTINCT FROM. After that, it reported the mismatch:
```
% make -C src/test/regress check-tests TESTS='test_setup opr_sanity'
# +++ regress check in src/test/regress +++
# initializing database system by copying initdb template
# using temp instance on port 58928 with PID 36865
ok 1 - test_setup 243 ms
# diff -U3 /Users/chaol/Documents/code/postgresql/src/test/regress/expected/opr_sanity.out /Users/chaol/Documents/code/postgresql/src/test/regress/results/opr_sanity.out
# --- /Users/chaol/Documents/code/postgresql/src/test/regress/expected/opr_sanity.out 2026-06-23 07:58:52
# +++ /Users/chaol/Documents/code/postgresql/src/test/regress/results/opr_sanity.out 2026-06-23 07:59:41
# @@ -488,9 +488,13 @@
# FROM pg_type t
# WHERE t.typarray = proargtypes[array_length(proargtypes, 1)-1])
# END IS DISTINCT FROM provariadic;
# - oid | provariadic | proargtypes
# ------+-------------+-------------
# -(0 rows)
# + oid | provariadic | proargtypes
# +---------------------------------------+-------------+--------------------------
# + pg_get_role_ddl(regrole,text) | text | [0:1]={regrole,text}
# + pg_get_tablespace_ddl(oid,text) | text | [0:1]={oid,text}
# + pg_get_tablespace_ddl(name,text) | text | [0:1]={name,text}
# + pg_get_database_ddl(regdatabase,text) | text | [0:1]={regdatabase,text}
# +(4 rows)
#
# -- Check that all and only those functions with a variadic type have
# -- a variadic argument.
not ok 2 - opr_sanity 8938 ms
1..2
# 1 of 2 tests failed.
```

After updating pg_proc.dat to change those "text" entries to "_text", the test passed.

See the attached patch for details.

[1] https://postgr.es/m/CANxoLDe_GOXyyZs7GN3VUJoT+o1DwqZGhid-TWWvS0D8d5ggYw@mail.gmail.com

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