Lock six-argument ImageResult construction

Adds a test asserting the pre-3.1 six-argument constructor still works
and defaults psnr to null, so the additive parameter cannot silently
become a breaking change.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: mathiasgrimm

tests/ClientTest.php

@@ -8,6 +8,7 @@
 use MathiasGrimm\GlimpsePhp\ForbiddenException;
 use MathiasGrimm\GlimpsePhp\ImageFormat;
 use MathiasGrimm\GlimpsePhp\ImageInfo;
+use MathiasGrimm\GlimpsePhp\ImageResult;
 use MathiasGrimm\GlimpsePhp\RateLimitException;
 use MathiasGrimm\GlimpsePhp\SizeEstimate;
 use MathiasGrimm\GlimpsePhp\Tests\Fixtures\Images;
@@ -69,6 +70,14 @@ function client(Factory $http, Closure|string|null $token = 'test-token', string
     expect(client($http)->optimize(Images::png())->psnr)->toBe(42.0);
 });
 
+test('the six-argument constructor stays valid and defaults psnr to null', function () {
+    // Locks backward compatibility: callers built against pre-3.1 must keep
+    // working without passing psnr.
+    $result = new ImageResult('bytes', 'jpg', 'image/jpeg', 100, 20, 10);
+
+    expect($result->psnr)->toBeNull();
+});
+
 test('convert sends optimize and quality when given', function () {
     $http = fakeHttp(['*/v1/convert' => Factory::response(fakeTransformResponse())]);