Method BuildSelectClause<TProps>(QueryContext<TProps>, IReadOnlyDictionary<string, FieldInfo>, SqlParameterCollector, IReadOnlySet<string>?, string)

Generates SELECT clause with DISTINCT support.
T-SQL doesn't have DISTINCT ON, so we use ROW_NUMBER() OVER (PARTITION BY ...).
Returns (selectClause, distinctPartitionExpr) for synchronization with ORDER BY.

public (string SelectClause, string? DistinctPartitionExpr) BuildSelectClause<TProps>(QueryContext<TProps> context, IReadOnlyDictionary<string, FieldInfo> fields, SqlParameterCollector collector, IReadOnlySet<string>? cteFieldNames = null, string cteAlias = "pvt_cte") where TProps : class, new()