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。

src/Jiaowu.Api/Infrastructure/Persistence/AppDbContext.cs

@@ -518,7 +518,7 @@ public sealed class AppDbContext(DbContextOptions<AppDbContext> options)
                 x.CourseSelectionOfferingId,
                 x.Status,
                 x.WaitlistedAt
-            });
+            }).HasDatabaseName("IX_CE_Offering_Status_WaitlistedAt");
             entity.HasOne(x => x.CourseSelectionOffering)
                 .WithMany(x => x.Enrollments)
                 .HasForeignKey(x => x.CourseSelectionOfferingId)

src/Jiaowu.Api/Infrastructure/Persistence/AppDbContextFactory.cs

@@ -12,7 +12,7 @@ public sealed class AppDbContextFactory : IDesignTimeDbContextFactory<AppDbConte
             ?? "Server=localhost;Port=3306;Database=jiaowu;User=__design_time__;Password=__not_used__;";
 
         var options = new DbContextOptionsBuilder<AppDbContext>()
-            .UseMySQL(connectionString)
+            .UseMySQL(MySqlConnectionStringPolicy.ForApplication(connectionString))
             .Options;
 
         return new AppDbContext(options);

src/Jiaowu.Api/Infrastructure/Persistence/DevelopmentSqliteMigrator.cs

@@ -1611,7 +1611,7 @@ public sealed class DevelopmentSqliteMigrator(
         ADD COLUMN "WaitlistedAt" TEXT NULL;
         """,
         """
-        CREATE INDEX IF NOT EXISTS "IX_CourseEnrollments_CourseSelectionOfferingId_Status_WaitlistedAt"
+        CREATE INDEX IF NOT EXISTS "IX_CE_Offering_Status_WaitlistedAt"
         ON "CourseEnrollments" ("CourseSelectionOfferingId", "Status", "WaitlistedAt");
         """
     ];