Fix 500 when page[limit]=0 is requested

[linkrobins]

Fixes #4773.

Problem

Any request to an offset-paginated endpoint with page[limit]=0 returns a 500:

GET /api/discussions?page[limit]=0

TypeError: Cannot assign null to property
Tobyz\JsonApiServer\Pagination\OffsetPagination::$limit of type int
in core/src/Api/Endpoint/Index.php:133

This is what was recorded on discuss in #4773. It reproduces on a clean install with no extensions.

Cause

RequestUtil::extractLimit() short-circuits with null on a falsy limit:

if (! $limit) {
    return null;
}

The string "0" is falsy, so it returns null before reaching the < 1 validation just below. That null is then assigned to OffsetPagination::$limit, which is a non-nullable int, so PHP throws.

The boundary confirms the mechanism — only the literal 0 hits this. -5, 00, 0.0 are all rejected with a clean 400, because they are truthy strings that reach the < 1 guard.

Fix

Use a strict === null check. A genuinely absent limit (no page[limit] and no default) still returns null, but an explicit page[limit]=0 now falls through to the existing < 1 guard and returns a 400, consistent with how negative limits are already handled.

Added unit tests in RequestUtilTest covering an explicit limit, capping at max, the default fallback, the no-limit null path, and rejection of 0 and negative values.