[REF] refactor file pool model and imports

- Refactor SnFilePool using freezed + sealed for consistency with other Sn models
- Add extension method listFromResponse for PoolService compatibility
- Update PoolService and utils to use SnFilePool
- Replace relative imports (../) with package imports for clarity and maintainability

Signed-off-by: Texas0295 <kimura@texas0295.top>

lib/models/file_pool.g.dart

@@ -0,0 +1,47 @@
+// GENERATED CODE - DO NOT MODIFY BY HAND
+
+part of 'file_pool.dart';
+
+// **************************************************************************
+// JsonSerializableGenerator
+// **************************************************************************
+
+_SnFilePool _$SnFilePoolFromJson(Map<String, dynamic> json) => _SnFilePool(
+  id: json['id'] as String,
+  name: json['name'] as String,
+  description: json['description'] as String?,
+  storageConfig: json['storage_config'] as Map<String, dynamic>?,
+  billingConfig: json['billing_config'] as Map<String, dynamic>?,
+  policyConfig: json['policy_config'] as Map<String, dynamic>?,
+  isHidden: json['is_hidden'] as bool?,
+  accountId: json['account_id'] as String?,
+  resourceIdentifier: json['resource_identifier'] as String?,
+  createdAt:
+      json['created_at'] == null
+          ? null
+          : DateTime.parse(json['created_at'] as String),
+  updatedAt:
+      json['updated_at'] == null
+          ? null
+          : DateTime.parse(json['updated_at'] as String),
+  deletedAt:
+      json['deleted_at'] == null
+          ? null
+          : DateTime.parse(json['deleted_at'] as String),
+);
+
+Map<String, dynamic> _$SnFilePoolToJson(_SnFilePool instance) =>
+    <String, dynamic>{
+      'id': instance.id,
+      'name': instance.name,
+      'description': instance.description,
+      'storage_config': instance.storageConfig,
+      'billing_config': instance.billingConfig,
+      'policy_config': instance.policyConfig,
+      'is_hidden': instance.isHidden,
+      'account_id': instance.accountId,
+      'resource_identifier': instance.resourceIdentifier,
+      'created_at': instance.createdAt?.toIso8601String(),
+      'updated_at': instance.updatedAt?.toIso8601String(),
+      'deleted_at': instance.deletedAt?.toIso8601String(),
+    };