* fix(filer/postgres): use pgx v5 API for PgBouncer simple protocol
In pgx/v5 the `prefer_simple_protocol` DSN parameter was removed, so
appending it to the connection string caused PgBouncer/PostgreSQL to
reject it as an unknown startup parameter:
FATAL: unsupported startup parameter: prefer_simple_protocol (SQLSTATE 08P01)
Parse the DSN with pgx.ParseConfig and, when pgbouncer_compatible is
set, configure DefaultQueryExecMode = QueryExecModeSimpleProtocol and
disable the statement/description caches. Register the config via
stdlib.RegisterConnConfig before sql.Open.
Fixes#9005
* refactor(filer/postgres): extract shared OpenPGXDB helper with cleanup
Extract the pgx v5 ParseConfig/RegisterConnConfig/sql.Open/Ping logic
into a shared postgres.OpenPGXDB helper used by both postgres and
postgres2 filer stores, eliminating ~60 lines of duplication.
The helper also unregisters the conn config via stdlib.UnregisterConnConfig
on every failure path (sql.Open error, Ping error) so we do not leak
entries in stdlib's global connection config map when initialization
fails.
* refactor(filer/postgres): use stdlib.OpenDB to avoid conn config leak
Switch OpenPGXDB from RegisterConnConfig + sql.Open("pgx", connStr) to
stdlib.OpenDB(*connConfig). The former leaks an entry in stdlib's global
conn config map on every successful initialization; stdlib.OpenDB takes
the config directly and keeps no global registration.
Addresses CodeRabbit review feedback on #9010.
