Add llms_can_user_bypass_restrictions filter for custom role bypass

[faisalahammad]

Description

Add the llms_can_user_bypass_restrictions filter to llms_can_user_bypass_restrictions() so custom and third-party roles (e.g. Memberium, User Role Editor) added to Unrestricted Preview Access can bypass enrollment, drip, and prerequisite restrictions.

The edit_post capability gate added in #1987 blocks roles that lack that capability. The filter gives developers an opt-in escape hatch. Default null preserves the existing behavior, including the #1974 security fix.

$can_bypass = apply_filters( 'llms_can_user_bypass_restrictions', null, $user, $post_id );
if ( null !== $can_bypass ) {
    return (bool) $can_bypass;
}

Fixes #2180

How has this been tested?

• composer tests -- --filter=test_llms_can_user_bypass_restrictions passes (29 assertions across 3 methods including 2 new filter tests).
• composer check-cs clean.
• Manual test plan in TESTING_INSTRUCTIONS.md.

Environment: LifterLMS dev environment, WordPress 6.x, PHP 7.4+.

Types of changes

• New feature (non-breaking change which adds functionality)

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.