将索引缩短为 IX_CE_Offering_Status_WaitlistedAt。

将候补迁移改为可重入：先检查列和索引是否存在，再执行 DDL。
附件中已经成功添加 WaitlistedAt、但索引创建失败的数据库可直接重试，不会报重复列。
运行时和 dotnet ef 连接统一开启 AllowUserVariables；Connector/NET 默认关闭该选项。Connector/NET 官方说明
增加所有 MySQL 索引名不得超过 64 字符的回归测试。

tests/Jiaowu.Api.Tests/MySqlMigrationTests.cs

@@ -2,6 +2,7 @@ using Jiaowu.Api.Infrastructure.Persistence;
 using Microsoft.EntityFrameworkCore;
 using Microsoft.EntityFrameworkCore.Infrastructure;
 using Microsoft.EntityFrameworkCore.Migrations;
+using MySql.Data.MySqlClient;
 
 namespace Jiaowu.Api.Tests;
 
@@ -27,9 +28,14 @@ public sealed class MySqlMigrationTests
         Assert.Contains("CREATE TABLE `WarningRules`", script);
         Assert.Contains("ADD `CheckInMethod` int NOT NULL DEFAULT 1", script);
         Assert.Contains("CREATE UNIQUE INDEX `IX_AttendanceSheets_CheckInToken`", script);
-        Assert.Contains("ADD `WaitlistedAt` datetime(6) NULL", script);
+        Assert.Contains("ADD COLUMN `WaitlistedAt` datetime(6) NULL", script);
         Assert.Contains(
-            "CREATE INDEX `IX_CourseEnrollments_CourseSelectionOfferingId_Status_WaitlistedAt`",
+            "CREATE INDEX `IX_CE_Offering_Status_WaitlistedAt`",
+            script);
+        Assert.Contains("@waitlisted_column_exists", script);
+        Assert.Contains("@waitlist_index_exists", script);
+        Assert.DoesNotContain(
+            "IX_CourseEnrollments_CourseSelectionOfferingId_Status_WaitlistedAt",
             script);
         Assert.Contains(
             "ADD `MaxCourseCount` int NULL",
@@ -86,6 +92,34 @@ public sealed class MySqlMigrationTests
         Assert.Contains(ids[1].ToString(), sql, StringComparison.OrdinalIgnoreCase);
     }
 
+    [Fact]
+    public void MySql_index_names_fit_the_server_identifier_limit()
+    {
+        using var db = new AppDbContext(CreateMySqlOptions());
+
+        var indexNames = db.Model.GetEntityTypes()
+            .SelectMany(x => x.GetIndexes())
+            .Select(x => x.GetDatabaseName())
+            .Where(x => !string.IsNullOrEmpty(x))
+            .ToList();
+
+        Assert.NotEmpty(indexNames);
+        Assert.All(indexNames, name => Assert.True(
+            name!.Length <= 64,
+            $"MySQL index name '{name}' has {name.Length} characters."));
+    }
+
+    [Fact]
+    public void Application_mysql_connections_allow_migration_user_variables()
+    {
+        var normalized = MySqlConnectionStringPolicy.ForApplication(
+            "Server=localhost;Database=jiaowu;User=test;Password=test;");
+
+        var builder = new MySqlConnectionStringBuilder(normalized);
+
+        Assert.True(builder.AllowUserVariables);
+    }
+
     private static DbContextOptions<AppDbContext> CreateMySqlOptions() =>
         new DbContextOptionsBuilder<AppDbContext>()
             .UseMySQL(