pgsql: Throw ERROR when publish_generated_columns is specified without

Throw ERROR when publish_generated_columns is specified without a value.

Previously, specifying the publication option 'publish_generated_columns'
without an explicit value would incorrectly default to 'stored', which is
not the intended behavior.

This patch fixes the issue by raising an ERROR when no value is provided
for 'publish_generated_columns', ensuring that users must explicitly
specify a valid option.

Author: Peter Smith <smithpb2250(at)gmail(dot)com>
Reviewed-by: vignesh C <vignesh21(at)gmail(dot)com>
Backpatch-through: 18, where it was introduced
Discussion: https://postgr.es/m/CAHut+PsCUCWiEKmB10DxhoPfXbF6jw5RD9ib2LuaQeA_XraW7w@mail.gmail.com

Branch
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REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/e5d04aedaf3a0e62dc22c42206bd258db24adddc

Modified Files
--------------
src/backend/commands/publicationcmds.c | 20 ++++++++++----------
src/test/regress/expected/publication.out | 15 ++++-----------
src/test/regress/sql/publication.sql | 7 ++-----
3 files changed, 16 insertions(+), 26 deletions(-)