fix(billing): 修复账单详情查询的数据库并发错误

问题:
- Npgsql.NpgsqlOperationInProgressException: A command is already in progress
- 在同一个数据库连接上，billReader 未释放就执行 paymentReader

根因:
- GetBillingDetailQueryHandler 中先查询账单并打开 billReader
- 读取账单数据后未释放 reader
- 直接在同一连接上执行支付记录查询，触发并发异常

解决方案:
- 将账单字段先读取到本地变量
- 主动 DisposeAsync 释放 billReader
- 再执行支付记录查询
- 最后用本地变量组装 BillingDetailDto

src/Application/TakeoutSaaS.Application/App/Billings/Handlers/GetBillingDetailQueryHandler.cs

@@ -46,14 +46,35 @@ public sealed class GetBillingDetailQueryHandler(
                     throw new BusinessException(ErrorCodes.NotFound, "账单不存在");
                 }
 
+                // 1.2 (空行后) 读取账单行数据到内存（释放 Reader，避免同连接并发执行命令）
+                var billingId = billReader.GetInt64(0);
+                var tenantId = billReader.GetInt64(1);
+                var tenantName = billReader.IsDBNull(2) ? string.Empty : billReader.GetString(2);
+                long? subscriptionId = billReader.IsDBNull(3) ? null : billReader.GetInt64(3);
+                var statementNo = billReader.GetString(4);
+                var billingType = (BillingType)billReader.GetInt32(5);
+                var status = (TenantBillingStatus)billReader.GetInt32(6);
+                var periodStart = billReader.GetDateTime(7);
+                var periodEnd = billReader.GetDateTime(8);
+                var amountDue = billReader.GetDecimal(9);
+                var discountAmount = billReader.GetDecimal(10);
+                var taxAmount = billReader.GetDecimal(11);
+                var amountPaid = billReader.GetDecimal(12);
+                var currency = billReader.IsDBNull(13) ? "CNY" : billReader.GetString(13);
+                var dueDate = billReader.GetDateTime(14);
                 DateTime? reminderSentAt = billReader.IsDBNull(15) ? null : billReader.GetDateTime(15);
                 DateTime? overdueNotifiedAt = billReader.IsDBNull(16) ? null : billReader.GetDateTime(16);
                 var notes = billReader.IsDBNull(17) ? null : billReader.GetString(17);
                 var lineItemsJson = billReader.IsDBNull(18) ? null : billReader.GetString(18);
+                var createdAt = billReader.GetDateTime(19);
                 long? createdBy = billReader.IsDBNull(20) ? null : billReader.GetInt64(20);
+                DateTime? updatedAt = billReader.IsDBNull(21) ? null : billReader.GetDateTime(21);
                 long? updatedBy = billReader.IsDBNull(22) ? null : billReader.GetInt64(22);
 
-                // 1.2 (空行后) 反序列化账单明细
+                // 1.3 (空行后) 主动释放账单 Reader，确保后续查询不会触发 Npgsql 并发命令异常
+                await billReader.DisposeAsync();
+
+                // 1.4 (空行后) 反序列化账单明细
                 var lineItems = new List<BillingLineItemDto>();
                 if (!string.IsNullOrWhiteSpace(lineItemsJson))
                 {
@@ -67,7 +88,7 @@ public sealed class GetBillingDetailQueryHandler(
                     }
                 }
 
-                // 1.3 (空行后) 查询支付记录
+                // 1.5 (空行后) 查询支付记录
                 var payments = new List<PaymentRecordDto>();
                 await using var paymentCommand = CreateCommand(
                     connection,
@@ -100,39 +121,36 @@ public sealed class GetBillingDetailQueryHandler(
                     });
                 }
 
-                // 1.4 (空行后) 组装详情 DTO
-                var amountDue = billReader.GetDecimal(9);
-                var discountAmount = billReader.GetDecimal(10);
-                var taxAmount = billReader.GetDecimal(11);
+                // 1.6 (空行后) 组装详情 DTO
                 var totalAmount = amountDue - discountAmount + taxAmount;
 
                 return new BillingDetailDto
                 {
-                    Id = billReader.GetInt64(0),
-                    TenantId = billReader.GetInt64(1),
-                    TenantName = billReader.IsDBNull(2) ? string.Empty : billReader.GetString(2),
-                    SubscriptionId = billReader.IsDBNull(3) ? null : billReader.GetInt64(3),
-                    StatementNo = billReader.GetString(4),
-                    BillingType = (BillingType)billReader.GetInt32(5),
-                    Status = (TenantBillingStatus)billReader.GetInt32(6),
-                    PeriodStart = billReader.GetDateTime(7),
-                    PeriodEnd = billReader.GetDateTime(8),
-                    AmountDue = billReader.GetDecimal(9),
-                    DiscountAmount = billReader.GetDecimal(10),
-                    TaxAmount = billReader.GetDecimal(11),
+                    Id = billingId,
+                    TenantId = tenantId,
+                    TenantName = tenantName,
+                    SubscriptionId = subscriptionId,
+                    StatementNo = statementNo,
+                    BillingType = billingType,
+                    Status = status,
+                    PeriodStart = periodStart,
+                    PeriodEnd = periodEnd,
+                    AmountDue = amountDue,
+                    DiscountAmount = discountAmount,
+                    TaxAmount = taxAmount,
                     TotalAmount = totalAmount,
-                    AmountPaid = billReader.GetDecimal(12),
-                    Currency = billReader.IsDBNull(13) ? "CNY" : billReader.GetString(13),
-                    DueDate = billReader.GetDateTime(14),
+                    AmountPaid = amountPaid,
+                    Currency = currency,
+                    DueDate = dueDate,
                     ReminderSentAt = reminderSentAt,
                     OverdueNotifiedAt = overdueNotifiedAt,
                     LineItemsJson = lineItemsJson,
                     LineItems = lineItems,
                     Payments = payments,
                     Notes = notes,
-                    CreatedAt = billReader.GetDateTime(19),
+                    CreatedAt = createdAt,
                     CreatedBy = createdBy,
-                    UpdatedAt = billReader.IsDBNull(21) ? null : billReader.GetDateTime(21),
+                    UpdatedAt = updatedAt,
                     UpdatedBy = updatedBy
                 };
             },