Connectors: Replace plugin.slug with plugin.file

[jorgefilipecosta]

Summary

• Replace plugin.slug with plugin.file (the full plugin file path, e.g. akismet/akismet.php) in connector registrations
• Simplify script module data: remove get_plugins() slug-to-file map, use file_exists() and is_plugin_active() directly
• Add isInstalled to the frontend plugin data
• Update PHPDoc for plugin type across registry and public API functions

Test plan

• Verify connectors page loads and shows correct plugin status
• Verify install and activate flows still work for AI provider plugins

[github-actions]

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[github-actions]

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