Fix JsonManipulator::addSubNode modifying a nested key with the same name - #13109

JsonManipulator::addSubNode() (used by addConfigSetting(), addProperty() and so composer config) looks for the existing child with a regex that is not anchored to the node's top level. If the same key name appears deeper inside the node first, that nested value gets replaced instead, and the intended top-level key is never added.

Reproduction with a composer.json containing:

{
    "extra": {
        "symfony": {
            "allow-contrib": true
        }
    }
}

composer config extra.allow-contrib false --json turns extra.symfony.allow-contrib into false and does not add extra.allow-contrib. Likewise addConfigSetting('php', ...) on a file with config.platform.php set overwrites the platform version instead of adding config.php.

The fix anchors $childRegex at the opening brace of the node and only allows skipping over complete "key": value members before the target key, so only direct children match. Nested objects are consumed whole by (?&json), so keys inside them can no longer match.

Added tests for addConfigSetting, addSubNode and addProperty with a nested same-name key; they fail without the change. tests/Composer/Test/Json passes and PHPStan reports no errors for the file.

Targeting 2.10 since the code is identical there and it's a bug fix; happy to retarget to main if preferred.