fix: correct index_attributes for indexes not on leading table columns

[oniani1]

Closes #1108

The /indexes query joins pg_attribute against the index relation but filters it with a.attnum = ANY(idx.indkey), and indkey holds the table's column numbers, not the index's. That only lines up when the index is on the table's leading columns, so composite indexes lose columns and an index on a later column drops out of the result entirely (and 404s on retrieve-by-id).

Switched the join to a.attnum > 0. The index relation's own attributes are already the index columns in index order, and the existing order by a.attnum keeps them in order, so this returns the full attribute list. It's the same guard columns.sql.ts already uses.

Added a test with a composite index and an index on a non-leading column.