Summary

• Allow the same nested array format as contain() for marshalling associations
• Uses CakePHP naming conventions to distinguish associations from options (no hardcoded list to maintain)

This enables writing:

$table->newEntity($data, [
    'associated' => [
        'Articles' => ['Comments', 'Tags', 'onlyIds' => true],
    ],
]);

Instead of requiring the explicit associated key at every level:

$table->newEntity($data, [
    'associated' => [
        'Articles' => [
            'associated' => ['Comments', 'Tags'],
            'onlyIds' => true,
        ],
    ],
]);

How it works

Uses CakePHP naming conventions to detect associations vs options:

• Association names start with uppercase (CamelCase): Users, Articles
• Option keys start with lowercase (camelCase): onlyIds, conditions
• Special data keys start with underscore: _joinData, _ids

This avoids maintaining a hardcoded list of known option keys that would need updating whenever new options are added.

Related to #18988 (6.x version).

If we wanted to harden this, we could go with the alternative option(s) in that 6.x PR (e.g. _options key for any custom options that are not assoc aliases), but then it couldnt be used in 5.next yet.