fix(item): normalize offsetSet to drop cross-version phpstan ignore

[louisbels]

Fix the failing CI on your roach-php#292 PR against roach-php/core.

Problem

The inline @phpstan-ignore offsetAccess.invalidOffset added to Item::offsetSet() is only triggered on PHP 8.5. On PHP 8.2/8.3/8.4, PHPStan doesn't report that error, so the ignore becomes unmatched and CI fails with:

ItemPipeline/Item.php
  63     No error with identifier offsetAccess.invalidOffset is reported on line 63.

That explains the 7 red jobs on the upstream PR roach-php#292.

Fix

Narrow the offset handling properly so no ignore is needed at all:

public function offsetSet(mixed $offset, mixed $value): void
{
    if ($offset === null) {
        $this->data[] = $value;
        return;
    }

    $this->data[$offset] = $value;
}

This handles the $array[] = $value case (where $offset is null per ArrayAccess semantics) explicitly, which makes the narrow type of the remaining $offset (int|string) enough for PHPStan.

Verified locally

PHPStan + full test suite on PHP 8.2, 8.4 and 8.5:

PHPStan: [OK] No errors
PHPUnit: Tests: 502, Assertions: 808, Warnings: 1, Deprecations: 5, Skipped: 16

Cheers.