Fix IDE0055 formatting errors in migration files

Add missing blank line between namespace declaration and using statements in three migration files to comply with C# formatting rules.

Co-authored-by: kamilbaczek <74410956+kamilbaczek@users.noreply.github.com>

Author: Copilot

Chapter-3-microservice-extraction/Fitnet.Contracts/Src/Fitnet.Contracts.Infrastructure/Database/Migrations/20230601184153_MakeSignedAtColumnNullable.cs

@@ -1,6 +1,7 @@
 #nullable disable
 
 namespace EvolutionaryArchitecture.Fitnet.Contracts.Data.Database.Migrations;
+
 using System;
 using Microsoft.EntityFrameworkCore.Migrations;
 

Chapter-3-microservice-extraction/Fitnet.Contracts/Src/Fitnet.Contracts.Infrastructure/Database/Migrations/20230601184328_AddCustomerIdColumn.cs

@@ -1,6 +1,7 @@
 #nullable disable
 
 namespace EvolutionaryArchitecture.Fitnet.Contracts.Data.Database.Migrations;
+
 using System;
 using Microsoft.EntityFrameworkCore.Migrations;
 

Chapter-3-microservice-extraction/Fitnet.Contracts/Src/Fitnet.Contracts.Infrastructure/Database/Migrations/20230624171216_ContractsAddColumnsToSupportContractExpiration.cs

@@ -1,8 +1,8 @@
 #nullable disable
 
 namespace EvolutionaryArchitecture.Fitnet.Contracts.Data.Database.Migrations;
-using
-    System;
+
+using System;
 using Microsoft.EntityFrameworkCore.Migrations;
 
 [ExcludeFromCodeCoverage]