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
------
master

Details
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https://git.postgresql.org/pg/commitdiff/e90e198e3f478ebc0b1fba0347ad9d76c48e7fa3

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(-)