fix: Load MX before instantiating the error mock controller

[hellopablo]

Problem

Early exceptions (before CodeIgniter's Router loads) can cascade into a second fatal when rendering the error view:

Class "MX_Controller" not found in vendor/nails/common/src/Common/Controller/Base.php:48

Seen in production against probe URLs with disallowed URI characters (e.g. /computer/(master)/script). URI filtering runs before the MX Router, so MX/Modules.php has not registered the MX_* autoloader. NailsMockController → App\Controller\Base → Nails\Common\Controller\Base extends MX_Controller then fatals.

Cause

ErrorHandler::instantiateMockController() loads CI_Controller, Security, Input, and Lang, then instantiates NailsMockController. It does not load Modular Extensions. MX_Controller is normally autoloaded via MX/Modules.php, which is only required from the MX Router.

Fix

• Require MX/Modules.php when MX_Controller is not already loaded, so the MX autoloader is in place before the mock controller is constructed.
• Wrap instantiateMockController() in try/catch (\Throwable) and fall back to halt() so a mock-controller failure never becomes an uncaught fatal.

Does not change URI filtering: production still halt()s disallowed characters as 404; non-prod still bubbles.

Follow-up

• Apps that miss the Nails URI subclass on Linux (e.g. NAILS_Uri.php vs NAILS_URI.php) should still rename that proxy so production 404s those probes without hitting this path. This PR is the safety net for any other pre-Router exception.
• Cherry-pick to develop if wanted (same pattern as fix: Pass charset/Security into ErrorHandler mock controller bootstrap #559 / fix: Pass charset/Security into ErrorHandler mock controller bootstrap #560).
• Bump consuming apps' composer.lock once this lands on feature/pre-new-admin.