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@gmail.com>
Reviewed-by: vignesh C <vignesh21@gmail.com>
Backpatch-through: 18, where it was introduced
Discussion: https://postgr.es/m/CAHut+PsCUCWiEKmB10DxhoPfXbF6jw5RD9ib2LuaQeA_XraW7w@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c9a5860f7a56cc639d6a73519b8b2a00d26d960c

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(-)