fix(symfony): check HttpClientTrait before loading the test client

[aleksejs1]

Q	A
Branch?	5.0
Tickets	Closes #8612
License	MIT
Doc PR	-

Since #8574, the container fails to compile when symfony/http-client is not installed. class_exists(Client::class) ran first and autoloaded Client, which uses HttpClientTrait. That is a fatal error, and the trait_exists(HttpClientTrait::class) guard that used to cover the case was never reached. The Client check now runs after the trait check.

Test: TestClientPassTest::testProcessWithoutHttpClient. It runs in a separate process and hides HttpClientTrait from the autoloaders, the same as an application without symfony/http-client.

Without the fix:

1) ApiPlatform\Symfony\Bundle\DependencyInjection\Compiler\TestClientPassTest::testProcessWithoutHttpClient
Error: Trait "Symfony\Component\HttpClient\HttpClientTrait" not found

/app/src/Test/Client.php:32
/app/src/Symfony/Bundle/DependencyInjection/Compiler/TestClientPass.php:29

With the fix: OK (4 tests, 6 assertions). tests/Symfony/Bundle/DependencyInjection/ passes: 68 tests, 2 skipped. php-cs-fixer and PHPStan report nothing on the two changed files.

I left CHANGELOG.md alone because its entries link to the commit hash.

[aleksejs1]

The five red checks are not caused by this change:

• PHPStan (PHP 8.5) and openapi (lowest, minimal-changes) fail the same way on the head of 5.0 (c092cd0). PHPStan reports 8 errors in OpenApiFactoryTest and PurgeHttpCacheListenerTest, and openapi fails testDeleteOperationWithDeserializeDocumentsRequestBody.
• PHPUnit (no deprecations) and PHPUnit (Symfony dev) fail on the deprecation of ApiPlatform\Symfony\Bundle\Test\ApiTestCase, triggered outside of tests. fix(metadata): use operation shortName for resources without ApiResource #8609 fails "no deprecations" the same way.

The jobs that run TestClientPassTest pass.