fix: Model handling of Entity $primaryKey casting

[kenjis]

Description
Fixes #7797
Supersedes #8271

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[kenjis]

batchUpdate() does not work because the following SQL cannot update the columns:

UPDATE `db_uuid`
INNER JOIN (
SELECT '<binary_uuid_value_1>' `id`, 'update1' `value` UNION ALL
SELECT '<binary_uuid_value_2>' `id`, 'update2' `value`
) `_u`
ON `db_uuid`.`id` = `_u`.`id`
SET
`db_uuid`.`value` = `_u`.`value`

The following patch makes the test pass:

--- a/system/Database/MySQLi/Builder.php
+++ b/system/Database/MySQLi/Builder.php
@@ -131,7 +131,7 @@ class Builder extends BaseBuilder
                 " UNION ALL\n",
                 array_map(
                     static fn ($value) => 'SELECT ' . implode(', ', array_map(
-                        static fn ($key, $index) => $index . ' ' . $key,
+                        static fn ($key, $index) => (is_string($index) ? '_binary' . $index : $index) . ' ' . $key,
                         $keys,
                         $value
                     )),

This makes the following SQL:

UPDATE `db_uuid`
INNER JOIN (
SELECT _binary'<binary_uuid_value_1>' `id`, _binary'update1' `value` UNION ALL
SELECT _binary'<binary_uuid_value_2>' `id`, _binary'update2' `value`
) `_u`
ON `db_uuid`.`id` = `_u`.`id`
SET
`db_uuid`.`value` = `_u`.`value`

See https://dev.mysql.com/doc/refman/8.0/en/string-literals.html

This is the same data type issue in batchUpdate() as the following case in PostgreSQL:
https://forum.codeigniter.com/showthread.php?tid=88871

[kenjis]

@codeigniter4/database-team Review, please.