Fix AuthTest to include required Livewire v4 headers (#499)

Livewire v4 added RequireLivewireHeaders middleware that returns 404
if requests don't include X-Livewire header and JSON content type.
Updated test POST requests to use postJson() with X-Livewire header.

For the 'doesnt use pulse middleware' test, changed assertion from
assertOk() to expect(status)->not->toBe(403), since Livewire v4 now
returns 404 for empty components — but the test's intent is verifying
Pulse auth (403) is not applied to non-Pulse Livewire requests.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>