test(config): expect json as canonical v2 serializer default

WorkflowsConfigTest was still asserting `Workflow\Serializers\Y::class` as
the canonical default. The v2 config already moved to `'json'`, so the test
could mask regressions in the json-default rollout.

Closes #336.

Author: durable-workflow-ops

tests/Unit/Config/WorkflowsConfigTest.php

@@ -22,7 +22,7 @@ public function testConfigIsLoaded(): void
             'stored_workflow_signal_model' => \Workflow\Models\StoredWorkflowSignal::class,
             'stored_workflow_timer_model' => \Workflow\Models\StoredWorkflowTimer::class,
             'workflow_relationships_table' => 'workflow_relationships',
-            'serializer' => \Workflow\Serializers\Y::class,
+            'serializer' => 'json',
             'prune_age' => '1 month',
             'webhooks_route' => env('WORKFLOW_WEBHOOKS_ROUTE', 'webhooks'),
         ];