GroupPurlProvider never sets its plugin ID, so getProviderId() returns NULL

Problem/Motivation

GroupPurlProvider::create() receives $configuration, $plugin_id and $plugin_definition but doesn't pass them to the constructor, and __construct() never calls PluginBase::__construct(). The plugin ID is never set, so getProviderId(), which returns getPluginId(), returns NULL. Subclasses that inherit create() have the same problem.

PURL's RequestSubscriber::onRequest() stores matched modifiers under the provider ID ($matches[$provider->getProviderId()]). When two providers both return NULL, their matches share the key '', and the second overwrites the first before hook_purl_matched_modifiers_alter() runs.

We hit this with a second provider that extends GroupPurlProvider to handle per-group custom domains. On a group's custom domain, a path starting with another group's alias matched both providers. The path-prefix match silently replaced the domain match, so the wrong group was activated.

Steps to reproduce

1. Enable purl and group_purl.
2. Run drush php:eval 'var_dump(\Drupal::service("purl.plugin.provider_manager")->createInstance("group_purl_provider")->getProviderId());'
3. Expected: string(19) "group_purl_provider". Actual: NULL.

Proposed resolution

• 3.0.x: Pass the plugin arguments from create() to the constructor and call parent::__construct(), following the standard plugin pattern. The patch updates GroupPurlProviderTest for the new constructor, adds a test that create() sets the plugin ID, and fixes the constructor docblock, which names GroupDomainPurlProvider.
• 2.2.x: Set the three plugin properties on the new instance in create() and leave the constructor alone. Changing the constructor signature in a minor release would break subclasses that call parent::__construct() with the current arguments.

API changes

3.0.x: GroupPurlProvider::__construct() takes three new leading parameters: $configuration, $plugin_id and $plugin_definition. Subclasses that call parent::__construct() need updating. 2.2.x: none.