update-core.php - plugin slug is not clean

[timse201]

The plugin slug we use to generate the link is not clean on update core. It includes the parent folder. e.g. /plugin/plugin.php. The API is smart enough to redirect to the en_us page.
On the plugins screen, we use a clean one.

$plugin_data->update->slug

​https://github.com/WordPress/wordpress-develop/blob/5e7fd3f6a7caf06e55b4b2781bd42a26165e5b3e/src/wp-admin/update-core.php#L570

The View version X details. link rendered by list_plugin_updates() on the WordPress Updates screen uses $plugin_data->update->slug directly when building the plugin-install.php?tab=plugin-information&plugin=... URL. In some cases this value is empty or includes the parent folder (e.g. plugin/plugin.php) instead of a clean plugin slug, producing a malformed details URL. The api.wordpress.org redirects mask this, but the URL is still incorrect and inconsistent with the link rendered on the Plugins screen.

This adds a small defensive fallback that derives the slug from the plugin directory name (matching the pattern already used in WP_Plugin_Upgrader) whenever the update response does not provide a clean slug.

Props timse201.
See #65218.

[audrasjb]

Removing trunk version as this is not going to be shipped with WP 7.0 but in the next releases.