feat(extensions): register sandboxed drivers in New Connection + activate via PluginRpcBridge

[ZhuchkaTriplesix]

Summary

Close the Discovery → Registration → Activation gap for installed database_driver extensions (e.g. ClickHouse).

Install already worked (LocalExtensionInstaller + SandboxPolicy), but drivers never appeared in New Connection because:

1. ExtensionManifest dropped contributions / capabilities on toJson() rewrite
2. Connection type list was a hardcoded enum of 5 built-ins
3. SDUI form / PluginRpcBridge were not wired into the connections feature

Scope

• Parse/persist contributions.drivers + capabilities
• SDUI host accepts key / boolean aliases (ClickHouse form schema)
• ConnectionRow.extension_id + driver_options (db v7)
• Dynamic New Connection list + SDUI save form
• ExtensionDriverSession → handshake → injectCredentials → db.connect → schema tree
• Follow-up: full SQL workspace / db.query for extension drivers

Note

Reinstall any previously installed driver packages after this lands — older installs had contributions stripped from disk.

[ZhuchkaTriplesix]

Implemented: sandboxed drivers appear in New Connection and activate via PluginRpcBridge.

Merged in PR #319.