fix: stop prompting "MariaDB plugin not installed" for lazy built-in drivers

[datlechin]

Summary

• Picking MariaDB (or any other lazy built-in driver) in the new-connection type chooser falsely showed "The MariaDB plugin is not installed. Would you like to download it from the plugin marketplace?" before the bundle had ever been activated.
• Root cause: PluginManager.isDriverLoaded(for:) only inspected driverPlugins (the in-memory map). Built-in drivers are registered lazily and only get inserted there on first use, so any first-click before activation routed through the marketplace prompt. MariaDB is the most visible case because its pluginTypeId resolves to "MySQL" via reverseTypeIndex, and driverPlugins["MySQL"] is empty until the MySQL bundle loads.
• Replaced isDriverLoaded(for:) with isDriverInstalled(for:), which also checks lazyDriverURLs[typeId] — populated by registerLazyPluginEntry for every type ID in the manifest, including aliases. Same fix removes the spurious "Not Installed" badge in DatabaseTypeChooserRow. Deleted the unused, identically-bodied isDriverAvailable(for:).
• Relaxed lazyDriverURLs from private to private(set) so the registration extension can read it; mutation still scoped to PluginManager.swift.

Test plan

• Fresh launch with no MySQL/MariaDB connection ever opened. Open the new-connection chooser, click MariaDB → connection form opens directly, no install alert.
• Same for MySQL, PostgreSQL, SQLite, ClickHouse, Redis (other lazy built-ins).
• Click a isDownloadable type the user has not installed (e.g. MongoDB on a clean profile) → install alert still shows correctly.
• Type chooser grid: verify "Not Installed" badge no longer appears under MariaDB / MySQL / PostgreSQL / etc., and still appears for genuinely uninstalled downloadable plugins.
• Existing MariaDB connection in the sidebar still connects on click.

[chatgpt-codex-connector]

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