Fix create_graph failing when ag_catalog is not in search_path

[NotHimmel]

Backport of a fix for the 1.7.0 line. Upstream: apache#2505 (PR), apache#2504 (issue).

Problem

With ag_catalog absent from search_path, creating a graph fails even for a
fully schema qualified call:

LOAD 'age';
SHOW search_path;          --  "$user", public

SELECT ag_catalog.create_graph('g');
-- ERROR:  operator class "graphid_ops" does not exist for access method "btree"

Same failure for create_vlabel(), create_elabel(), the CSV loader, and a
CREATE/MERGE clause that mentions a label for the first time — the last one
fails in the middle of a running query. GRANT USAGE ON SCHEMA ag_catalog and
GRANT EXECUTE do not help, because the operator class exists and is simply
not visible.

Root cause

create_index_on_column() passed the operator class as an unqualified name, so
ResolveOpClass() resolved it through the caller's search_path:

index_col->opclass = list_make1(makeString("graphid_ops"));

This was the only unqualified name on that code path. Column types are reached
by OID (GRAPHIDOID, AGTYPEOID), and _graphid(), agtype_build_map() and
_label_id() are already built with list_make2(makeString("ag_catalog"), ...).

The unqualified name arrived with the id column indexes in apache#2117, so
this is a regression against 1.6.0, where create_label() built no index and
resolved no name through search_path.

Testing

PostgreSQL 18.4, built from source: # All 31 tests passed.

Reverting the change and re-running reproduces the error above, so the
one-line change is what fixes it.

Note this does not remove the documented search_path requirement for
querying a graph — some names in the query path are still resolved
unqualified. It only restores create_graph() to not depending on it.