fix: show rule identity instead of backend identity in SHOW POOLS

[douyux]

Problem

SHOW POOLS and SHOW POOLS_EXTENDED report route->id.database and
route->id.user in the database and user columns. These fields
represent the backend connection identity — they are overridden by
storage_db / storage_user directives during routing.

When multiple frontend routing rules map to the same backend credentials,
all affected routes produce rows with identical (database, user) values,
making them indistinguishable. This causes two concrete problems:

1. Observability gap — operators cannot correlate pool metrics back to
   specific routing rules.
2. Prometheus label collision — duplicate label sets corrupt metric
   series and break scraping.

Fix

Replace route->id.database / route->id.user with
route->rule->db_name / route->rule->user_name in
od_console_show_pools_add_cb() (sources/console.c).

These fields always hold the frontend rule identity (the name as
written in the config file), which is unique per route. For default
rules they resolve to the literal strings "default_db" /
"default_user".

The length fields are updated accordingly: rule->db_name_len and
rule->user_name_len are assigned via strlen() and do not include the
null terminator, so the previous - 1 adjustment is dropped.

Behaviour change

Scenario	Before	After
No storage_db / storage_user	client DB / user	rule DB / user (same)
storage_user = backend_user configured	backend_user	rule user name
Multiple rules → same backend	duplicate rows	distinct rows per rule

Operators who relied on SHOW POOLS to inspect the actual backend
credentials should use SHOW SERVERS instead, which continues to reflect
the real backend connection identity.

Fixes #1279.