Add runtime migration creation and application

[thromel]

Summary

Implements #37342: Allow to create a migration and apply it without recompiling.

This adds support for creating and applying migrations at runtime using Roslyn compilation, enabling scenarios like .NET Aspire and containerized applications where recompilation isn't possible.

New Features

CLI:

dotnet ef database update --add MigrationName [--output-dir <DIR>] [--namespace <NS>]

Programmatic API:

var result = context.Database.CreateAndApplyMigration("MigrationName");
// or with options
var result = context.Database.CreateAndApplyMigration("MigrationName", new RuntimeMigrationOptions
{
    PersistToDisk = true,
    OutputDirectory = "Migrations",
    DryRun = false
});

Architecture

Component	Purpose
IMigrationCompiler / CSharpMigrationCompiler	Roslyn-based compilation of scaffolded migrations
IDynamicMigrationsAssembly / DynamicMigrationsAssembly	Decorator that merges runtime-compiled migrations with static ones
IRuntimeMigrationService / RuntimeMigrationService	Orchestrates scaffold → compile → register → apply flow
RuntimeMigrationOptions	Configuration for persistence, output directory, namespace, dry-run
RuntimeMigrationResult	Contains migration ID, applied status, SQL commands, file paths

Limitations

• Requires dynamic code generation (incompatible with NativeAOT) - marked with [RequiresDynamicCode]
• C# only (no VB.NET/F# support initially)

Test plan

• Unit tests for CSharpMigrationCompiler (5 tests)
• Unit tests for DynamicMigrationsAssembly (12 tests)
• All existing EFCore.Design.Tests pass (1210 tests)
• All existing EFCore.Relational.Tests pass (1386 tests)
• Integration tests with actual database (SQLite: 8 tests, SQL Server: 7 tests)
• CLI functional tests (2 tests)

Fixes #37342

[thromel]

Note on SQL Server Integration Tests

The RuntimeMigrationSqlServerTest tests are marked with [SqlServerCondition(SqlServerCondition.IsNotAzureSql | SqlServerCondition.IsNotCI)] and are skipped in CI. This follows the same pattern used by MigrationsInfrastructureSqlServerTest.

Why these tests are skipped in CI:

• They require creating fresh databases dynamically for each test to properly test the migration flow from scratch
• The Helix CI environment has SQL Server available but with limited permissions configured for shared/pre-configured databases
• Tests that use SqlServerTestStore.CreateInitializedAsync with dynamic database names don't work in the CI SQL Server setup

Test coverage is still maintained:

• Core runtime migration logic is covered by unit tests in EFCore.Design.Tests (which run in CI)
• The SQL Server integration tests run locally for developers with SQL Server configured
• SQLite integration tests in EFCore.Sqlite.FunctionalTests also validate the end-to-end flow

This is consistent with how other complex migration infrastructure tests handle CI limitations.