fix:생산지시 생성 시 다중 담당자(assignee_ids) 저장 누락 수정

- CreateProductionOrderRequest에 assignee_ids 배열 validation 추가
- OrderService::createProductionOrder에 work_order_assignees 저장 로직 추가
- 담당자 유무에 따른 status 분기 (pending/unassigned) 적용

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/Http/Requests/Order/CreateProductionOrderRequest.php

@@ -22,6 +22,8 @@ public function rules(): array
             'priority' => 'nullable|string',
             'process_type' => ['nullable', Rule::in(WorkOrder::PROCESS_TYPES)],
             'assignee_id' => 'nullable|integer|exists:users,id',
+            'assignee_ids' => 'nullable|array',
+            'assignee_ids.*' => 'integer|exists:users,id',
             'team_id' => 'nullable|integer|exists:departments,id',
             'scheduled_date' => 'nullable|date',
             'memo' => 'nullable|string',