Fix upsert for HasOne when existing record has no ID in input

When using `upsert` on a HasOne relation without providing an ID, the
existing implementation always created a fresh model via `$relation->make()`.
This caused duplicate records to be created instead of updating the
existing related record.

Now checks if a record already exists via `$relation->first()` before
falling back to creating a new model instance.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: spawnia

src/Execution/Arguments/NestedOneToOne.php

@@ -36,7 +36,8 @@ public function __invoke($model, $args): void
         if ($args->has('upsert')) {
             $upsertModel = new ResolveNested(new UpsertModel(new SaveModel($relation)));
 
-            $upsertModel($relation->make(), $args->arguments['upsert']->value);
+            $existing = $relation->first();
+            $upsertModel($existing ?? $relation->make(), $args->arguments['upsert']->value);
         }
 
         if ($args->has('delete')) {

tests/Integration/Execution/MutationExecutor/HasOneTest.php

@@ -423,6 +423,54 @@ public function testUpdateAndUpsertHasOne(string $action): void
         ]);
     }
 
+    /** @dataProvider existingModelMutations */
+    #[DataProvider('existingModelMutations')]
+    public function testUpdateAndUpsertExistingHasOneWithoutID(string $action): void
+    {
+        $task = factory(Task::class)->create();
+        $this->assertInstanceOf(Task::class, $task);
+
+        $task->post()
+            ->save(
+                factory(Post::class)->create(),
+            );
+
+        $this->graphQL(/** @lang GraphQL */ <<<GRAPHQL
+        mutation {
+            {$action}Task(input: {
+                id: 1
+                name: "foo"
+                post: {
+                    upsert: {
+                        title: "bar"
+                    }
+                }
+            }) {
+                id
+                name
+                post {
+                    id
+                    title
+                }
+            }
+        }
+        GRAPHQL)->assertJson([
+            'data' => [
+                "{$action}Task" => [
+                    'id' => '1',
+                    'name' => 'foo',
+                    'post' => [
+                        'id' => '1',
+                        'title' => 'bar',
+                    ],
+                ],
+            ],
+        ]);
+
+        // Assert that the existing post was updated, not a new one created
+        $this->assertSame(1, Post::count());
+    }
+
     /** @dataProvider existingModelMutations */
     #[DataProvider('existingModelMutations')]
     public function testUpdateAndDeleteHasOne(string $action): void