pgsql: Be precise about when ALTER SUBSCRIPTION needs conninfo.

Be precise about when ALTER SUBSCRIPTION needs conninfo.

Decide early whether the original conninfo is needed so that errors
happen consistently.

Addresses finding 12 in report from linked discussion.

Co-authored-by: Shlok Kyal <shlok(dot)kyal(dot)oss(at)gmail(dot)com>
Reported-by: Noah Misch <noah(at)leadboat(dot)com>
Reviewed-by: Hayato Kuroda (Fujitsu) <kuroda(dot)hayato(at)fujitsu(dot)com>
Reviewed-by: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
Discussion: https://postgr.es/m/20260710195902.4f.noahmisch%40microsoft.com
Backpatch-through: 19

Branch
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REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/ae420d66e72051b7ceab49d14f990094f1c8c6b4

Modified Files
--------------
src/backend/commands/subscriptioncmds.c | 79 +++++++++++++++++++-----------
src/test/regress/expected/subscription.out | 8 +++
src/test/regress/sql/subscription.sql | 9 ++++
3 files changed, 68 insertions(+), 28 deletions(-)