Regression: database existence check in HistoryRepository no longer goes through DatabaseCreator

[roji]

As part of #34115, the implementation of HistoryRepository.Exists was changed to no longer use the DatabaseCreator:

Old Exists

public virtual bool Exists()
	=> Dependencies.DatabaseCreator.Exists()
		&& InterpretExistsResult(
			Dependencies.RawSqlCommandBuilder.Build(ExistsSql).ExecuteScalar(
				new RelationalCommandParameterObject(
					Dependencies.Connection,
					null,
					null,
					Dependencies.CurrentContext.Context,
					Dependencies.CommandLogger, CommandSource.Migrations)));

New Exists

public virtual bool Exists()
	=> InterpretExistsResult(
		Dependencies.RawSqlCommandBuilder.Build(ExistsSql).ExecuteScalar(
			new RelationalCommandParameterObject(
				Dependencies.Connection,
				null,
				null,
				Dependencies.CurrentContext.Context,
				Dependencies.CommandLogger, CommandSource.Migrations)));

This breaks PG, and likely other databases as well - it assumes that one can connect/login with the connection string containing the non-existing database, and then find out whether the database exists via the query. But in PG you can't connect to a non-existing database.

As a result, NpgsqlDatabaseCreator implements its Exists by simply trying to connect, and interpreting "database doesn't exist" behavior to return false etc.