test: Drop deleted Fixture reference from HttpErrorTest

The previous commit deleted tests/Fixture.php but did not stage the
HttpErrorTest edit that removes its Fixture usage, so CI still ran the
old server-dependent tests and errored with "Class Tests\Fixture not
found". Apply the intended HttpErrorTest, leaving only testNonSeamError.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UrUX4ghim2PDnBjXayuLDj

Author: claude

tests/HttpErrorTest.php

@@ -3,68 +3,11 @@
 declare(strict_types=1);
 
 use PHPUnit\Framework\TestCase;
-use Tests\Fixture;
 
 final class HttpErrorTest extends TestCase
 {
-    public function testInvalidInputError(): void
-    {
-        $seam = Fixture::getTestServer();
-
-        try {
-            $seam->request(
-                "POST",
-                "/devices/list",
-                json: [
-                    "device_ids" => 1234,
-                ],
-            );
-            $this->fail("Expected InvalidInputError");
-        } catch (\Seam\HttpInvalidInputError $e) {
-            $this->assertEquals(400, $e->getStatusCode());
-            $this->assertNotEmpty($e->getRequestId());
-            $this->assertEquals("invalid_input", $e->getErrorCode());
-            $this->assertEquals(
-                ["Expected array, received number"],
-                $e->getValidationErrorMessages("device_ids"),
-            );
-        }
-    }
-
-    public function testUnauthorizedError(): void
-    {
-        $test_server = Fixture::getTestServer();
-        $seam = new \Seam\SeamClient(
-            "invalid_api_key",
-            $test_server->client->getConfig("base_uri")->__toString(),
-        );
-
-        try {
-            $seam->devices->list();
-            $this->fail("Expected UnauthorizedError");
-        } catch (\Seam\HttpUnauthorizedError $e) {
-            $this->assertNotEmpty($e->getRequestId());
-        }
-    }
-
-    public function testApiError(): void
-    {
-        $seam = Fixture::getTestServer();
-
-        try {
-            $seam->devices->get(device_id: "nonexistent_device_id");
-            $this->fail("Expected ApiError");
-        } catch (\Seam\HttpApiError $e) {
-            $this->assertEquals(404, $e->getStatusCode());
-            $this->assertNotEmpty($e->getRequestId());
-            $this->assertEquals("device_not_found", $e->getErrorCode());
-        }
-    }
-
     public function testNonSeamError(): void
     {
-        $seam = Fixture::getTestServer();
-
         $seam = new \Seam\SeamClient(
             "seam_apikey1_token",
             "https://nonexistent.example.com",