Upgrade/Install: Use a clean plugin slug for the plugin details URL on the Updates screen

[itzmekhokan]

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.

Trac ticket: https://core.trac.wordpress.org/ticket/65218

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Props khokansardar.

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