Limit number of memberships shown in student dashboard

[faisalahammad]

Description

The My Memberships tab (/my-account/my-memberships/) lists every membership a student belongs to with no cap and no pagination. lifterlms_template_my_memberships_loop() now caps the list at 500 by default. The limit can be changed with a new filter:

add_filter( 'llms_my_memberships_loop_limit', function( $limit, $student ) {
	return 10; // or 0 for no limit
}, 10, 2 );

This mirrors the existing pattern in lifterlms_template_my_courses_loop(), which already caps its list at 500 the same way.

Fixes #2442

How has this been tested?

Added PHPUnit coverage in tests/phpunit/unit-tests/functions/class-llms-test-functions-templates-dasbhoard.php:

• Membership count under the limit renders unchanged.
• A 0 filter value disables the cap entirely.
• A filter that lowers the limit actually trims the rendered list.

Ran composer tests -- --filter LLMS_Test_Functions_Templates_Dashboard (11/11 passing) and composer check-cs (clean).

Manually tested on a local install: enrolled a test student in several memberships, confirmed the dashboard tab renders all of them normally, then lowered the filter to 1 and confirmed only 1 membership rendered, then set it to 0 and confirmed all memberships rendered again.

Checklist:

• This PR requires and contains at least one changelog file.
• My code has been tested.
• My code passes all existing automated tests.
• My code follows the LifterLMS Coding & Documentation Standards.