Removed Obsolete attributes from FirstAsync and Exceptions class. Changed FirstAsync return type to non-nullable Task<T> and updated its logic to throw the provided exception when the sequence is empty, instead of returning null.
Added FirstAsync<T, TException> as an obsolete extension method for Task<IEnumerable<T>>. This method returns the first element or throws a custom exception if the result is null, using a provided factory delegate. Intended for legacy .NET projects.
Marked TaskExtensions class and all its methods as [Obsolete] with guidance to implement Mediator behaviors instead. Added new [Obsolete] extension methods for null/empty checks and chaining. Introduced an [Obsolete] Exceptions class with factory methods for common exceptions. All changes are intended for legacy or transitional use only.
