Fix RuntimeMigration tests by skipping table creation in test store

Migration tests need to start with an empty database (no tables) so they
can test creating and applying migrations from scratch. The default test
stores call EnsureCreatedResilientlyAsync() which creates all tables,
causing "table already exists" errors when tests try to run migrations.

This fix creates custom test store factories that:
- SQLite: Uses SqliteTestStore.GetExisting() which sets seed=false
- SQL Server: Creates a custom SqlServerTestStore subclass that overrides
  InitializeAsync to skip table creation

Each test still calls CleanDatabase() to drop any existing tables before
running, ensuring a clean state for each test.

Author: thromel

test/EFCore.SqlServer.FunctionalTests/RuntimeMigrationSqlServerTest.cs

@@ -3,6 +3,7 @@
 
 using System.Data.Common;
 using Microsoft.EntityFrameworkCore.SqlServer.Design.Internal;
+using Microsoft.EntityFrameworkCore.TestUtilities;
 
 namespace Microsoft.EntityFrameworkCore;
 
@@ -60,7 +61,7 @@ FROM sys.foreign_keys f
     public class RuntimeMigrationSqlServerFixture : RuntimeMigrationFixtureBase
     {
         protected override ITestStoreFactory TestStoreFactory
-            => SqlServerTestStoreFactory.Instance;
+            => RuntimeMigrationSqlServerTestStoreFactory.Instance;
 
         protected override List<string> GetTableNames(DbConnection connection)
         {
@@ -75,4 +76,44 @@ protected override List<string> GetTableNames(DbConnection connection)
             return tables;
         }
     }
+
+    /// <summary>
+    /// Custom test store factory for RuntimeMigration tests that creates an empty database
+    /// without seeding (creating tables). This is necessary because migration tests need
+    /// to start with an empty database to test creating and applying migrations from scratch.
+    /// </summary>
+    private class RuntimeMigrationSqlServerTestStoreFactory : SqlServerTestStoreFactory
+    {
+        public static new RuntimeMigrationSqlServerTestStoreFactory Instance { get; } = new();
+
+        private RuntimeMigrationSqlServerTestStoreFactory()
+        {
+        }
+
+        public override TestStore GetOrCreate(string storeName)
+            => new RuntimeMigrationSqlServerTestStore(storeName);
+    }
+
+    /// <summary>
+    /// A SqlServerTestStore subclass that skips seeding the database with tables.
+    /// The InitializeAsync override creates the database but doesn't call EnsureCreated.
+    /// </summary>
+    private class RuntimeMigrationSqlServerTestStore : SqlServerTestStore
+    {
+        public RuntimeMigrationSqlServerTestStore(string name)
+            : base(name)
+        {
+        }
+
+        protected override Task InitializeAsync(
+            Func<DbContext> createContext,
+            Func<DbContext, Task> seed,
+            Func<DbContext, Task> clean)
+        {
+            // Skip the base implementation which calls EnsureCreatedResilientlyAsync()
+            // For RuntimeMigration tests, we want an empty database without tables
+            // so we can test creating and applying migrations from scratch.
+            return Task.CompletedTask;
+        }
+    }
 }

test/EFCore.Sqlite.FunctionalTests/RuntimeMigrationSqliteTest.cs

@@ -4,6 +4,7 @@
 using System.Data.Common;
 using Microsoft.Data.Sqlite;
 using Microsoft.EntityFrameworkCore.Sqlite.Design.Internal;
+using Microsoft.EntityFrameworkCore.TestUtilities;
 
 namespace Microsoft.EntityFrameworkCore;
 
@@ -18,7 +19,7 @@ protected override Assembly ProviderAssembly
     public class RuntimeMigrationSqliteFixture : RuntimeMigrationFixtureBase
     {
         protected override ITestStoreFactory TestStoreFactory
-            => SqliteTestStoreFactory.Instance;
+            => RuntimeMigrationSqliteTestStoreFactory.Instance;
 
         protected override List<string> GetTableNames(DbConnection connection)
         {
@@ -33,4 +34,21 @@ protected override List<string> GetTableNames(DbConnection connection)
             return tables;
         }
     }
+
+    /// <summary>
+    /// Custom test store factory for RuntimeMigration tests that doesn't seed the database.
+    /// This is necessary because migration tests need to start with an empty database
+    /// so they can test creating and applying migrations from scratch.
+    /// </summary>
+    private class RuntimeMigrationSqliteTestStoreFactory : SqliteTestStoreFactory
+    {
+        public static new RuntimeMigrationSqliteTestStoreFactory Instance { get; } = new();
+
+        private RuntimeMigrationSqliteTestStoreFactory()
+        {
+        }
+
+        public override TestStore GetOrCreate(string storeName)
+            => SqliteTestStore.GetExisting(storeName);
+    }
 }