Defer OperatorObservabilityRepository binding to app

The v2 customization matrix documents
Workflow\V2\Contracts\OperatorObservabilityRepository as a user-replaceable
container binding for Waterline/history-export integrations. The service
provider used singleton() to register the package default, which silently
overwrote any implementation an application bound before the workflow
provider registered (for example from an AppServiceProvider that loaded
first in bootstrap/providers.php).

Switch to singletonIf() so the package default only binds when no
implementation has been registered. Applications retain full control
whether they bind before or after the package provider.

Add testOperatorObservabilityRepositoryBindingDefersToAppBinding to
cover the pre-bound case: a custom implementation registered before
WorkflowServiceProvider::register() must survive and resolve.

Author: durable-workflow-ops

src/Providers/WorkflowServiceProvider.php

@@ -52,7 +52,7 @@ public function register(): void
     {
         $this->mergeConfigFrom(__DIR__ . '/../config/workflows.php', 'workflows');
 
-        $this->app->singleton(
+        $this->app->singletonIf(
             OperatorObservabilityRepository::class,
             DefaultOperatorObservabilityRepository::class,
         );

tests/Unit/Providers/WorkflowServiceProviderTest.php

@@ -17,6 +17,7 @@
 use Workflow\Providers\WorkflowServiceProvider;
 use Workflow\Serializers\Serializer;
 use Workflow\States\WorkflowPendingStatus;
+use Workflow\V2\Contracts\OperatorObservabilityRepository;
 use Workflow\V2\Enums\RunStatus;
 use Workflow\V2\Enums\TaskStatus;
 use Workflow\V2\Enums\TaskType;
@@ -79,6 +80,23 @@ public function testProviderBootAllowsConfiguredInstanceModelWithExplicitRelatio
         $this->assertTrue(true);
     }
 
+    public function testOperatorObservabilityRepositoryBindingDefersToAppBinding(): void
+    {
+        // The customization matrix documents OperatorObservabilityRepository
+        // as a user-replaceable container binding. When an application binds
+        // its own implementation before the package service provider runs
+        // (e.g. from an app service provider loaded earlier, or via test
+        // setup), the package must not overwrite that binding.
+        $custom = $this->createMock(OperatorObservabilityRepository::class);
+
+        $this->app->offsetUnset(OperatorObservabilityRepository::class);
+        $this->app->singleton(OperatorObservabilityRepository::class, static fn () => $custom);
+
+        (new WorkflowServiceProvider($this->app))->register();
+
+        $this->assertSame($custom, $this->app->make(OperatorObservabilityRepository::class));
+    }
+
     public function testProviderMergesV2DefaultsIntoLegacyPublishedConfig(): void
     {
         config()->set('workflows', [