fix: include cross-JOIN ON referenced columns in count subquery for MySQL - #878

Summary

Fix for issue #824 (multi-JOIN count pagination). When HEAD/count pagination is used with multiple JOINs, the subquery for each JOIN table only selected its own ON key (on.getKey()). If another JOIN's ON clause references a column of this table, that column was missing from the subquery, causing an Unknown column error.

Example (from the issue):

SELECT count(*) AS `count` FROM `Inventory` AS `Inventory`  
   LEFT JOIN ( SELECT `code` FROM `Location_info` ) AS `Location_info` ON `Location_info`.`code` = `Inventory`.`location_code`  
   LEFT JOIN ( SELECT `id` FROM `Warehose_info` ) AS `Warehose_info` ON `Warehose_info`.`id` = `Location_info`.`warehouse_id`

Location_info's subquery only selects code, but the second ON clause references Location_info.warehouse_id.

Change

In AbstractSQLConfig.parseJoin (MySQL-family databases only), when building the subquery column list for a JOIN table, also collect the targetKey of any ON clause (from any JOIN in the list) whose target table/alias matches the current JOIN table. Uses a LinkedHashSet to keep insertion order and deduplicate.

PostgreSQL (and other non-MySQL databases) keep their existing SELECT * fallback, which already covers this case.

Tests

Added AbstractSQLConfigJoinCountTest with 2 cases:

• PostgreSQL: subqueries fall back to SELECT * (regression guard).
• MySQL: Location_info subquery must include both code (its own ON key) and warehouse_id (referenced by the second JOIN's ON).

Full suite: 38 tests, 0 failures.

fixes #824