Move SQL JSON helper function setup from runtime initialization into EF Core migrations

[Copilot]

JsonFunction.InitializeAsync was creating provider-specific SQL JSON helper functions at app startup instead of as part of schema evolution. This moves that setup into EF Core migrations for each SQL dialect so migrated databases consistently get the required functions through the normal migration path.

• Provider migrations

  • Add app migrations for MySQL, Postgres, and SQL Server that create the JSON helper functions from the existing embedded SQL resources.
  • Implement corresponding Down behavior so function lifecycle stays owned by migrations.

• Shared migration plumbing

  • Add a small shared helper to load the embedded SQL resources and apply them in a provider-appropriate way:
    • Postgres executes the full script.
    • MySQL / SQL Server split and apply individual statements.

• Runtime initialization

  • Remove the app-level SqlDialectInitializer<TContext> registration from EF service wiring so app startup no longer recreates these functions outside migration execution.

• Migration coverage

  • Extend provider migration tests to assert the helper functions are available after migrations complete, rather than only asserting that some migration ran.

protected override void Up(MigrationBuilder migrationBuilder)
{
    JsonFunctionMigration.Create(
        migrationBuilder,
        typeof(PostgresDialect),
        "Squidex.Providers.Postgres.json_function.sql",
        splitStatements: false);
}